Late Pleistocene and Holocene environmental changes at Lake Albano and Lake Nemi (central Italy) as...

P. Guilizzoni and F. Oldfield (Guest Editors)Palaeoenvironmental Analysis of Italian Crater Lake and Adriatic SedimentsMem. Ist. ital. Idrobiol., 55: 119-148, 1996

Late Pleistocene and Holocene environmental changes atLake Albano and Lake Nemi (central Italy) as indicatedby algal remains

Dave B. RYVES1, Viv J. JONES1, Piero GUILIZZONI2, Andrea LAMI2,Aldo MARCHETTO2, Richard W. BATTARBEE1, Roberta BETTINETTI2 andEmma C. DEVOY1

1Environmental Change Research Centre, University College, London, U.K.2CNR Istituto Italiano di Idrobiologia, 28048 Verbania Pallanza, Italy

ABSTRACT

Three cores from two crater lakes in central Italy, Lake Albano (cores PALB 94-1E andPALB 94-6B) and Lake Nemi (PNEM 94-1E), collected as part of the interdisciplinaryPALICLAS project, are discussed here. Core PALB 94-1E (13.875 m long, taken at a depth of70 m) covers the last ca 30 kyr years, PALB 94-6B (8.40 m, at 30 m depth) spans from ca 17-26 kyr BP, and PNEM 94-1B (9.15 m at 30 m depth) from ca 11 kyr BP to the present. Thesewere analysed for organic matter, CaCO3, biogenic silica, carbon, nitrogen, preserved pig-ments, diatoms and chrysophytes. Primary productivity was reconstructed from pigments (allcores) and total phosphorous from diatom assemblages (PALB 94-1E and PNEM 94-1B). Inpre-Holocene sequences from Lake Albano (PALB 94-1E and PALB 94-6B) there are severalwell-defined high productivity events in full generally oligotrophic glacial conditions, as in-ferred from pigments and diatom assemblages and supported by other proxies. The balanceof all the evidence about these basal oscillations is strongly towards a climatic forcing.Trends in productivity are similar for many proxies in the Holocene records of Lake Albanoand Lake Nemi (PALB 94-1E and PNEM 94-1B). The early Holocene is characterised byvery high productivity at both sites. Trophic levels throughout much of the period of humanactivity are lower but remain relatively high and stable at both lakes. It is only in most recenttimes that inferred productivity levels exceeding those of the early Holocene are found. Theseproxy records show great internal consistency in their response to distinct, but inter-related,aspects of the lake-catchment system, and reflect major changes in these systems since thelate Pleistocene, particularly in lake productivity. This study demonstrates the value and ne-cessity for a multi-proxy, multi-core approach in resolving complex palaeolimnologicalquestions of both anthropogenic and natural environmental change.

Key words: diatoms, pigments, chrysophytes, crater lakes, central Italy, late Pleistocene,Holocene

1. INTRODUCTION

The Mediterranean area is a dynamic transitional zone linking the north Africanclimate system to western Europe. This region could play a key role in understandingthe present and future direction of continental climatic trends (Kutzbach & Street-

D.B. Ryves et al.120

Perrot 1985). A knowledge of past climatic variations in this area can provide thebasis of such prediction.

The crater lake sediment sequences of central and southern Italy can be viewed asarchives of environmental change integrating regional climatic and local catchmentfactors, and more recently anthropogenic impacts at both scales. Several sites havenow been investigated, for example, lakes Monterosi and Baccano (Hutchinson1970), Vico (Frank 1969; Mergeai 1995), Monticchio (Watts 1985; Watts et al.1996a, 1996b), Martignano (Kelly & Huntley 1991), and the dry lakes Valle di Cas-tiglione, Piana del Fucino and Lagaccione (Follieri et al. 1988; Magri & Follieri1989; Follieri et al. 1993). The links between lacustrine sedimentation and climaticand tectonic history have also been explored in lakes Bolsena, Bracciano, Albanoand Nemi (Niessen et al. 1993), and the recent human impact of lake eutrophication(Masaferro et al. 1993; Lami et al. 1994).

The PALICLAS project employed high resolution (up to decadal or better),multi-proxy biological and geochemical records to investigate the impact of climatic,and anthropogenic, change on aquatic (lacustrine and marine) ecosystems, and theircatchments, over the last 30 kyr. The database now assembled is unique both interms of its temporal resolution and the range of mutually-independent sediment-based proxy records analysed, with which to assess the nature, rate and amplitude ofecosystem response to climatic change in central Italy (Oldfield 1996, this volume).

Lacustrine algal remains (e.g., diatoms, pigments and chrysophyte scales) canserve as valuable indicators of past environments (e.g., Berglund 1986; Smol 1990)and can act as indices of present and past trophic conditions (Leavitt 1993). They areuseful for revealing successional changes in glacial and postglacial ecology,limnology and climate, especially changes in aquatic productivity (Sanger 1988). Inconjunction with other proxy-records, pigments can provide information on the bal-ance between allochthonous and autochthonous organic contributions to the sedi-ment, redox conditions, and past periods of meromixis (Lami et al. 1994). The sili-ceous valves of diatoms, and chrysophyte cysts are generally abundant and well pre-served in lake sediments. Diatoms are sensitive ecological indicators and have beenwidely used to reconstruct changes in pH (Battarbee & Renberg 1990), salinity (Fritz1989) and nutrients (e.g., Bennion 1994). Approaches similar to these can be appliedto chrysophyte microfossils (Smol 1990).

In this paper we report data and palaeoenvironmental reconstructions obtainedfrom the analysis of algal remains from cores from Lake Albano (cores PALB 94-1Eand PALB 94-6B) and from Lake Nemi (PNEM 94-1B).

2. METHODS

A seismic survey of lakes Albano and Nemi identified a range of sites for coring(Chondrogianni et al. 1996a, this volume) and several cores were taken in June 1994using a Kullenberg piston corer (Kelts et al. 1986). The cores discussed here werecollected at a water depth of 70 m (PALB 94-1E, 13.875 m long) and 30 m (PALB94-6B, 8.40 m long) in Lake Albano and at 30 m in Lake Nemi (PNEM 94-1B, 9.15m long). Reference is also made to a deep water Holocene core from Lake Albano,

Environmental changes at lakes Albano and Nemi 121

PALB 94-3A (120 m water depth, 12.20 m long). Core segments, ca 1 m long, werestored at 4 °C until processing. For the analyses, 0.5-2 cm thick slices were sampledthroughout the cores.

2.1. Organic matter, CaCO3, BSiO2, carbon, nitrogen and pigments

Organic matter and CaCO3 content were measured by loss on ignition (LOI) at550 °C and 950 °C respectively. Carbon and nitrogen analyses were made using aCN elemental analyzer (Carlo Erba).

Biogenic silica (BSiO2) content was derived from total silica measured by X-rayfluorescence according to Robinson et al. (1993). Total pigments (chlorophyll pluschlorophyll derivatives, CD; and carotenoids, TC) were extracted with 90% acetone.For comparison with previous studies, chlorophyll derivatives were calculated asabsorbance units per gram organic matter (Wetzel 1970; Lami et al. 1994). Totalcarotenoids were expressed as mg per gramme organic matter (Züllig 1982). Specificalgal pigments were determined by ion pairing, reverse-phase HPLC (Beckman) andexpressed as nMoles per g organic matter (Lami et al. 1994).

Pigment preservation, and thus the quality of pigment data, can be assessed by avariety of sedimentary indices (Guilizzoni et al. 1992, Lami et al. 1994). Conditionsfor pigment preservation are generally good in both Lake Albano and Lake Nemi.

2.2. Diatoms and chrysophyte cysts

Diatoms and chrysophytes were prepared using standard digestion procedures(Battarbee & Kneen 1982; Battarbee 1986; Renberg 1990). Diatom frustules andchrysophyte cysts were identified and counted using both light (×1250) and electronmicroscopy. Cyst types were classified following Duff et al. (1995), and diatomswere identified using a range of standard floras (e.g., Krammer & Lange-Bertalot1986-1993). Diatom concentrations are reported as number of valves per grammedry matter (109 valves g dm-1).

2.3. Data analysis and trophic reconstruction

Profiles from each data set were split into zones using CONISS (CONstrainedIncremental Sum of Squares cluster analysis), with square root transformation tooptimize the signal to noise ratio. Zones were constructed independently for eachproxy using variables indicated in each figure. Samples with missing data values forsome variables were excluded for the purposes of zonation.

Primary in-lake palaeoproductivity was independently reconstructed from bothpreserved pigments and diatom assemblages, according to Guilizzoni et al. (1983)and Wunsam & Schmidt (1995) respectively. Two important caveats must be at-tached in applying the diatom transfer function method of Wunsam & Schmidt(1995) to these data. Firstly, there is a taxonomic issue, as certain of the diatom taxaare not unequivocally identifiable as those recorded by Wunsam & Schmidt (1995),despite taxonomic harmonisation and slide comparison of key taxa. Secondly, there


are no-analogue problems, of two types. Specifically, it was not possible to recon-struct TP for samples with significant proportions of Cyclotella sp.1 (most of thepre-Holocene); and generally there still remain questions of applicability of a trans-fer function developed for modern alpine, pre-alpine lakes to a crater lake sequence.

3. RESULTS AND DISCUSSION

Two cores from Lake Albano (PALB 94-1E and PALB 94-6B) and one fromLake Nemi (PNEM 94-1B) are discussed here. Chronologies for all these cores wereobtained using a variety of techniques and are discussed in detail in Chondrogianniet al. (1996a, this volume) and Oldfield (1996, this volume). All ages quoted are incalibrated years.

Core PALB94-1E was chosen as the master core for the lacustrine record withinthe PALICLAS project, as it spans the period from ca 30 kyr BP to the present, al-though a sediment hiatus representing ca 3400 years was discovered in the Holoceneinterval (see Chondrogianni et al. 1996a, Lowe et al. 1996, this volume). Thepossibility of a second hiatus close to the Glacial/Holocene transition is underevaluation (Oldfield 1996, this volume). This hiatus may have involved the loss of asmuch as ca 40 cm.

Apart from a 20 cm veneer of Holocene sediment, core PALB94-6B was depos-ited entirely within the pre-late Glacial, while core PNEM94-1B from Lake Nemi isa Holocene sequence.

3.1. Lake Albano core PALB94-1E

Many of the geochemical, pigment and biological proxy records in core PALB94-1E are remarkably consistent, despite reflecting different aspects of the lake-catchment system. The most obvious parallel trend in many of the analyses is a majorchange from ca 7-5 m, from the onset of the late Glacial and the transition into theHolocene.

3.1.1. Geochemistry and pigments

Figure 1 shows the geochemical data for core PALB 94-1E, with zones GI-GV,and figure 2 selected pigment profiles for this core split into seven zones (PI-PVII).

The concentrations of organic matter (LOI) in the sediment layers in zones GIand GII (Full Glacial; Fig. 1), are distinctly lower (mean content of 6.43% d.m.) thanin the upper zones (mean value of 19.53% d.m.). However, carbon content isvariable within zone GII (13.59 m - ca 10 m) where CaCO3 (10-25% d.m.) concen-trations are also high.

Accurate interpretation of the palaeoecological record, based on pigment data,depends to a large extent on the source of organic matter incorporated within lakesediments. The ratios of chlorophyll derivatives to total carotenoids (CD:TC) andC:N provide important clues to allochthonous versus autochthonous sources of or-ganic matter. Allochthonous plant detritus, although poor in total pigments, tends to


be relatively richer in chlorophyll derivatives than carotenoids because carotenoidsare more susceptible to oxidation than the phaeo-chlorophyll derivatives. Largevariations in the CD:TC ratios as registered, for example, in zone PI (13.5-11 m),zones PVI and PVII (Fig. 2), indicate a shift from aerobic to anaerobic decomposi-tion at the sediment-water interface, or a predominantly allochthonous source oforganic matter. However, allochthonous inputs of pigments seem to be negligible inLake Albano, because it receives water mainly from the atmosphere and underwatersprings (Lami et al. 1994; Boni et al. 1995). The rather low values of the C:N ratios(Fig. 1), although variable, support this assertion.

Fig. 1. Lake Albano core PALB 94-1E. Selected physical and geochemical parameters. Or-ganic matter = loss on ignition (LOI); BSiO2 = biogenic silica; % w.w. = % wet weight; %d.m. = % dry matter. CONISS zones GI-GV are shown. Ages given are calibrated 14C years.

The earliest glacial sediment (13.5-7 m; zone PI) generally also contains lowconcentrations of pigments (Fig. 2) indicating low productivity. The mean value ofchlorophyll derivatives (CD), total carotenoids (TC) and ß-carotene for this part ofthe core are 5.70 units, 0.13 mg g-1 o.m., and 13.09 nM g-1 o.m., respectively (Fig.2). These values are well below the averages of eutrophic holomictic and dimicticlakes (Guilizzoni et al. 1982; Guilizzoni & Lami 1992). As preservation would begood in anoxic to near anoxic bottom waters, the pigment concentrations in the coresuggest that throughout 13.5 m to 8 m the lake remained largely oligotrophic (exceptfor a few levels) and did not develop intense phytoplankton blooms. This maysuggest that during this period conditions were oxic in the water column and at themud surface for much of the time. Low concentrations of green and purple sulphurphotosynthetic bacteria are observed between 23 kyr BP and 17 kyr BP (10-7 m).


However, there are some interesting exceptions in the lowest ca 350 cm of thecore, within zone PI. In this interval, occasional intense phytoplankton and photo-synthetic sulphur bacteria blooms developed and virtually all the indicators of or-ganic productivity show sharp peaks. At least five major clear and rapid events werenoted: 1297-1282 cm; 1227-1213 cm; 1139-1129 cm; 1079-1055 cm and 1020-1005cm (Fig. 2). Lithological analysis suggests mostly uniform clastic sedimentation overthis section (Chondrogianni et al. 1996a, this volume). Assuming a constantsedimentation rate between 17 kyr BP and 28 kyr BP, we calculated a sedimentationrate of 0.048-0.052 cm yr-1 for this full glacial period. Thus, the average duration ofthese oscillations ranged between ca 200 and ca 500 years.

Fig. 2. Lake Albano core PALB 94-1E. Selected preserved sedimentary algal and bacterialpigments and inferred primary productivity (for explanation see text). CONISS zones PI-PVIIare shown. Units are per gram organic matter (g LOI-1). U = absorbance units. Ages given arecalibrated 14C years.

Mean CD, TC and ß-carotene concentrations (95.56 U. g-1 o.m., 1.59 mg g-1 o.m.and 275.91 nM g-1 o.m. respectively) for the remaining part of the core (7 m - sur-face; zones PII-PVII) are similar to the concentrations found in productive lakes withanoxic bottom waters (Guilizzoni et al. 1982; Guilizzoni & Lami 1992). A relativelygreater allochthonous contribution of organic matter is evident during the Holocene.

Intense but sporadic algal blooms during the Holocene can be identified between530 cm and 380 cm (zones PIII and PIV). Sedimentation rates estimated to rangefrom 0.050 to 0.091 cm yr-1 imply cycles of between ca 200 and 700 years. From theRoman period onward (top ca 137 cm; zone PVI-PVII; Fig. 2; Lowe et al. 1996, thisvolume) the pigment data suggest increasing trophic status. Increased primary


productivity in these periods promoted anoxia in the bottom waters, conducive topigment preservation. Sediments in Lake Albano from ca 6 m - 5.5 m and from 2 mupward accumulated in bottom waters that were often anoxic or close to being an-oxic.

In the upper part of the core (above 6 m) the carotenoids lutein (Chlorophyceae),zeaxanthin (not shown) and echinenone (Cyanobacteria) were present in high con-centrations (Fig. 2). Diadinoxanthin, one of the characteristic diatom carotenoids,was detected in low quantities, but no fucoxanthin (the second specific diatomcarotenoid) was detected. These pigments are known to be highly susceptible to de-composition (Leavitt 1993).

The presence of the photosynthetic sulphur bacterial carotenoids okenone andisorenieratene in zones PIII and PIV is significant (Fig. 2). Isorenieratene is a spe-cific product of green photosynthetic bacteria (Chlorobiaceae, e.g. Chromatium sp.)and okenone of purple photosynthetic sulphur bacteria (Chromatiaceae, e.g. Chloro-bium sp.), and are also abundant throughout the Holocene (Fig. 2; Guilizzoni et al.1996a; Lami et al. 1996). These organisms proliferate in strictly anoxic conditions inlake waters or at the water/sediment interface in the presence of H2S. Phaeophor-bides pigments are also abundant in this section, and at the surface (Fig. 2) and areclosely associated with zooplankton grazing activity (Leavitt et al. 1989; Lami et al.1994; Manca et al. 1996, this volume).

3.1.2. Diatoms

The diatom flora of Lake Albano and Lake Nemi is dominated throughout thecore sequence by centric taxa, several forms of which were difficult to identifypositively. Problems arose due to confusing and sometimes conflicting descriptionsin the literature (particularly small Stephanodiscus spp) and the occurrence of taxawhich cannot readily be found in the literature, and which may be extinct (e.g., Cy-clotella sp.1). These are problems of identification and no analogue respectively, andboth cause problems of interpretation and environmental reconstruction.

After an exchange of light micrographs and slide comparisons between the re-search groups of UCL-ECRC and CNR-III a working taxonomy was agreed to pro-mote consistency between both groups. This harmonisation also involved other dia-tomists outside PALICLAS, who were consulted during the research. A more de-tailed account of these taxonomic issues will be presented elsewhere.

Lake Albano has been dominated by planktonic taxa (often over 80%; Fig. 3)throughout its history except at the very base of the core (zone DI: 1325-1376 cm)where epiphytic, benthic and other littoral forms briefly peak (especially Achnanthesminutissima and vars.). Floral diversity increases above this level, notably in thelower half of the core below 650 cm. Diatom concentrations in this zone are ex-tremely low and may reflect low productivity and high dilution by detrital material.


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Zone DII (650-1325 cm) is dominated by the centric Cyclotella sp.1, often over40% and exceeding 70% for significant sections below 1200 cm. This taxon essen-tially disappears above zone DII, being replaced in zone DIII by significant butfluctuating proportions of other Cyclotella and Stephanodiscus taxa. Fragilaria andSynedra taxa constitute an important part of the total assemblage throughout thiszone at this site, usually accounting for about 25% of the total, but up to about 70%at 1070 cm. Within this zone there is significant fluctuation in the planktonic as-semblage with the sudden but persistent appearance of Asterionella formosa (1060-880 cm, up to 25% at 920 cm), Cyclotella ocellata (as high as 25% between 1025-860 cm: a large morphotype, in contrast to valves in the uppermost zones) andFragilaria reicheltii (between 10-20% from 840-700 cm). Other taxa appear moresporadically, for example Cyclotella cf comensis (40% at 1025 cm and 15% at 700-650 cm) and Cyclotella pseudostelligera (up to 35% but only found between 793.5-801.5 cm). Of most interest, however, are the peaks (up to 50%) in small Stephano-discus species towards the base of PALB94-1E, particularly from 1270-1290 cm and1210-1230 cm. High resolution work has shown these replace Cyclotella sp.1completely in both sections (Devoy 1996).

Zone DIII (510-650 cm) marks the rise to dominance of smaller centric forms,notably Stephanodiscus minutulus (often at 20-25%) and Cyclotella ocellata (with apeak of 50% at 570 cm). Other centrics are found, but Cyclotella sp.1 has essentiallydisappeared. Fragilaria and Synedra taxa are also important in the assemblage ofthis zone, accounting for 25-50% of the total, although the planktonic signal is neverless than 40% throughout this period.

The assemblage of zone DIV (380-510 cm) is characterised by a diatom signalprimarily consisting of simultaneous planktonic blooms of several small Stephano-discus taxa (particularly S. parvus and S. minutulus, but both S. hantzschii and S.medius are significant constituents). Relative proportions of Stephanodiscus parvusand S. minutulus are not less than 15% and 30% respectively, with maxima of 50%(S. parvus at 420 cm) and 75% (S. minutulus at 460 cm). Non-planktonic forms,such as Achnanthes clevei and Amphora pediculus although a minor (generally lessthan 5%) but regular component of the assemblage in zones DIII and DIV, are muchless frequently encountered above zone DIII (510 cm).

Diatom abundance in zone DIV is highly variable but the highest values are co-incident with the largest peaks in Stephanodiscus parvus and S. minutulus.

The uppermost zone DV (0-380 cm) consists of an assemblage consisting largelyof Cyclotella ocellata (commonly 40-50% and up to 80% at 320 cm), Cyclotella sp.2(10-25%, the larger values principally at the base and top of the zone) andStephanodiscus species (primarily S. minutulus, values increasing above 310 cm to30-50%). Non-planktonic forms are in very low percentages (less than 10%), exceptfor the uppermost 50 cm or so, while planktonic forms generally account for the restof the assemblage, and never fall below 70% of the total count.

Zone DV is the only zone for which there is a clearer trend in diatom abundance,values tending to increase from the base to the top of the zone. The highest values ofthe whole core are found in this zone at 19.5 cm (24.4 109 g dm-1) and 161 cm (13


109 g dm-1), and concentrations throughout are generally high as Cyclotella ocellataand Stephanodiscus minutulus are co-dominant.

The Holocene/Pleistocene boundary is placed at around 570 cm and coincideswith the complete disappearance of the planktonic centric, Cyclotella sp.1 above thislevel. Its stratigraphic distribution appears linked to lake conditions peculiar to theGlacial/late Glacial period. The mesotrophic taxon Asterionella formosa also has itsgreatest abundance in the lower half of the core, especially from 875-1050 cm (zone2). This to a large extent coincides with the large valve morphotype of Cyclotellaocellata, although the ecological or environmental significance of this taxon's mor-phological variation is unknown.

Fragilaria and Synedra are relatively important in both zone DII and zone DIII,particularly from about 1000-1150 cm and 510-580 cm. Although these mainlybenthic taxa are found at many late Glacial sites associated with lower and fluctu-ating lake level (Digerfeldt 1975), higher percentage abundance within a diatom as-semblage does not necessarily imply lower water level. Changes in the representationof these genera in the sediment record merely reflect differences in the absolute fluxof valves from different sedimentary sources. Higher relative abundance may reflecta decrease of planktonic productivity, an increase in importance of littoral habitat,closer proximity to the littoral source area, or enhanced sediment transport fromdiatom-rich littoral areas. Littoral habitat may increase due to greater abundance ofmacrophyte hosts, clearer water (greater depth of littoral zone), an increase in extentof littoral habitat by a drop in lake level, or catchment sedimentary input shallowingnear-shore areas. A fall in lake level is one possible explanation of several for thediatom change in this section (cf Manca et al. 1996, this volume), which diatomanalysis alone cannot fully resolve.

Although small Stephanodiscus taxa are linked to highly eutrophic waters incontemporary systems, it should be noted that there are significant peaks of theseforms in the lower 4 m of the core, during full glacial conditions. These may be in-directly related to short periods of warm conditions (see pigment paragraph) or peri-ods of geothermal activity affecting nutrient input, availability and cycling within thelake system (Belis et al. 1996). The disappearance of Cyclotella sp.1 during theseperiods (Devoy 1996) also points to lake conditions quite unlike those whichoccurred during the majority of the pre-Holocene, and may provide support for thehypothesis that this taxon was dependent on certain conditions in the lake only ful-filled during Glacial times (for example, extensive ice cover). Without validatedmodern occurrences of the taxon, and therefore knowledge of its ecology, suchconjectures remain speculative.

Above zone DII, the record is dominated by small Stephanodiscus taxa, and in-creasingly in zone DV, by Cyclotella ocellata, which is found in significant numbersthroughout the top 10 m of the core. This suggests that during the early Holocene thelake became much more productive, especially in zone DIV, and has remained so tothe uppermost levels, where there may be evidence for a slight reduction in eutrophy(as Cyclotella taxa dominate at the expense of small Stephanodiscus).

Pollen evidence implies the beginning of catchment clearance at 390 cm (ca 7500yr BP, but there is uncertainty over this date; see Lowe et al. 1996, this volume) and


the start of the Roman period at ca 150 cm (2000 yr BP); 390 cm marks the point atwhich the Stephanodiscus taxa, especially Stephanodiscus parvus, decline. Previouswork has suggested that the onset of human interference on catchments (such as theclearance of forests and their replacement with agricultural land use) is linked toincreasing nutrient inputs to and elevated nutrient levels in surface waters. The risein small Stephanodiscus taxa before this period may be independently linked to theclimatic amelioration of the early Holocene.

Despite the misgivings related to TP reconstruction, earlier results suggest thattotal phosphorous levels (TP in µg l-1; Fig. 3) were generally high throughout theHolocene, but especially in the early Holocene (520-450 cm; 500 cm is dated at9920 yr BP). From this point TP levels fall unevenly until rising at about 320 cm to amore sustained high level in the mid-Holocene (295-300 cm dated at 3860 yr BP).This increase is somewhat after the pollen-inferred onset of anthropogenic catchmentclearance at 390 cm (ca 7500 yr BP). From this level TP remains high until theuppermost 20 cm. Only a few levels with small percentages of ecologically unknowntaxa have TP values reconstructed in the pre-Holocene, during Stephanodiscusepisodes and at the very base of the core (dominated by Achnanthes minutissima). Itis likely TP values were low or very low when the lake had just formed, and was lowthroughout the full Glacial except for these periods of Stephanodiscus blooms.

Diatom valve concentrations broadly agree with the trend in the reconstructed TPabove 650 cm. The greatest concentrations are generally from 400-500 cm andabove 300 cm; while the sections from 300-400 cm, 650-500 cm and the uppermost10-20 cm have generally lower values. Diatom valve concentrations per gramme drymatter weight are only indirect proxies for diatom productivity, as sediment accu-mulation rates will not be constant in time and diatom biovolumes and biomass aredependent on valve dimensions and species-specific carbon content per volume.

3.1.3. Chrysophyte cysts

Remains of these algae are rare in Lake Albano, especially when compared withthe number of diatom frustules (Fig. 4). The stability and low value (<0.1-2.4%) ofthe Chrysophyte:diatom ratio (Ch:Di) throughout the sediment sequence (with oneexception; see below) is probably a consequence of lake conditions favouring a richdiatom phytoplankton community despite major environmental changes in the lakeduring the late Pleistocene and Holocene. From 56 samples examined, reliable cystcounts were only obtained in 33 samples with a Ch/Di ratio higher than 1.0-1.2%.About 110 cyst types were defined, most of them represented by a single specimen.Only morphotypes 32 and 17 accounted for more than 5% and 10% in any sample,respectively.

With the exception of one sample at the base (1357 cm), the lowermost section ofthe core (zone CI, and at around 1317-1337 cm) was very poor in cysts (Ch/Di ratiolower than 0.1%). In this section, the diatom assemblage is dominated by epiphyticand non-planktonic forms, mainly belonging to the genus Achnanthes. These signalsin a clearly laminated sediment suggests a very low lake level.


Fig. 4. Lake Albano core PALB 94-1E. Relative abundance of main chrysophyte cyst types,with the ratio of cysts:diatoms. CONISS zones CI-CVb are shown. Ages given are calibrated14C years.

At 1357 cm, and between ca 1300 and 527 cm (zone CII and CIII), the cyst as-semblage is dominated (42%-83%) by small cysts characteristic of deep, coldoligotrophic lakes, described as types No. 4, 5, 33, 83 and 224 by Duff et al. (1995),indicating a low trophic status (collectively referred as “oligotrophic cyst”). How-ever, some samples in this zone (at 678.5 cm, 933 cm, 995.5 cm, 1059.5 cm and1219.5 cm) show lower percentages of oligotrophic cysts, and higher percentage ofcysts No. 46 and 146. These samples correspond to the peaks in pigment concentra-tion and small Stephanodiscus species interpreted as pulses of increased lake pro-ductivity. Further peaks in lake productivity were detected by pigment and diatomanalyses at around 1010 cm, 1134 cm and 1290 cm, but in the corresponding levelsthe Ch/Di ratio was too low to allow reliable cyst counts.

A single sample with a relatively high Ch/Di of 8.5% (794.5 cm, ca 20 kyr BP)has a high percentage of cyst No. 31, reported as cold-tolerant (Duff & Smol 1989),with a wide tolerance for trophic status (Duff & Smol 1989) and pH (Zeeb & Smol1993). This sample is also characterised by an anomalous diatom flora, with a highpercentage of Cyclotella pseudostelligera, a species which is confined to this section(see section 3.1.2). The occurrence of the two signals at the same core depth wouldsuggest a sudden change in the planktonic habitat, but this is not supported by otherevidence from any other record. Cyst No. 31 also peaks in zones CIV and CVb.

Zone CIV (527-379 cm) is characterised by a lower percentage (3% to 46%) ofoligotrophic cysts, in agreement with the high productivity detected in this zone by


pigment and diatom analysis. Zone CVa (379-112.5 cm) was poor in cysts, withCh/Di ratio generally below 0.5%. In two samples counted, the percentage ofoligotrophic cysts is intermediate (40%) between the values of zone CII-CIV. Thesample at 112.5 cm is richer in cysts (Ch/Di 1.3%), with a high percentage of typeNo. 140, produced by an epiphytic species. Finally, at the top of the core, zone CVbcontains samples with low percentages of oligotrophic cysts.

3.2. Core PALB 94-6B

Core PALB 94-6B was correlated with core PALB 94-1E, using planktonic dia-tom assemblages. The top and bottom of PALB 94-6B are correlated to around 700cm and 1000-1050 cm, respectively, in core PALB 94-1E. This means that ca 8 m ofcore PALB 94-6B is represented by about 3 m in core PALB 94-1E, covering thetime span between ca 17 kyr BP and ca 25 kyr BP (Juggins 1996). This ca 8 kyrinterval was also confirmed by the varve counts in core PALB 94-6B(Chondrogianni et al., 1996b; this volume). Pollen evidence implies the Lake Albanocatchment was largely non-forested steppe, in agreement with a Glacial age for thiscore (Lowe et al. 1996, this volume).

Core PALB 94-6B allows a comparison between sites within Lake Albano whichdiffer in water depth and location relative to littoral and catchment input sources.Not only are inputs affected, but processes affecting important sedimentarycharacteristics such as water column stratification (affecting redox conditions at thewater/sediment interface for example) and sedimentation regimes. It also provides asalutary lesson in within-lake accumulation consistency and sedimentation rate, asthere appears to be little Holocene accumulation at this site, but much more rapidsedimentation when material was being deposited during the full Glacial. CorePALB 94-6B thus provides an opportunity to examine the full Glacial period at amuch higher resolution than PALB 94-1E.


The shallow depth of this core site may have affected processes of pigmentdeposition and microfossil abundance, extensively reviewed by Sanger (1988) andLeavitt (1993), particularly oxygen conditions in the water column. Sedimentedpigments may not be preserved well if oxic conditions persist at the sediment-waterinterface, even when productivity is high.

The geochemical and pigment profiles can be divided into several zones, whichreflect cyclic fluctuations in lake trophic status (zones GI-GV and PI-PIX respec-tively; Figs 5, 6). Zones PII, PIV, PVI and PVIII are periods of high inferred pri-mary productivity, whilst the opposite is true of the intervening zones. These highproductivity periods last an estimated ca 90-650 yr (Fig. 6). Within these high pro-ductivity zones there is considerable fluctuation in levels of primary productivity.

Zones of high productivity are probably related to warmer periods of relativelyhigher water levels and could be explained by blooms of planktonic or benthic algae.Such an interpretation of these zones is supported by other proxies. Sediments are


well laminated (implying anoxia) with high values of CaCO3 (biologically inducedprecipitation; Fig. 5). The high positive carbon isotope values (Chondrogianni et al.1995), magnetic data (Rolph et al. 1996, this volume), ostracod and cladoceraabundance (Manca et al. 1996, this volume) all agree with this hypothesis.

Fig. 5. Lake Albano core PALB 94-6B. Selected physical and geochemical parameters. Or-ganic matter = loss on ignition (LOI); BSiO2 = biogenic silica; % w.w. = % wet weight; %d.m. = % dry matter. CONISS zones GI-GV are shown. Ages given are calibrated 14C years.

By contrast, these peaks do not correlate with sections of abundant moss remains.The development of mosses (several species of Drepanocladus spp; preliminaryidentification by Prof. C. Andreis, University of Milan, and Prof. R. Ochyra, PolishAcademy of Sciences, Lubicz) is probably associated with colder and very low waterlevels (see Chondrogianni et al. 1996a, this volume).

The most distinctive feature of the pre-Holocene pigment data is the presence ofa number of cyanobacteria taxa, as well as several groups of purple sulphur andpurple non-sulphur photosynthetic bacteria (Guilizzoni et al. 1996a; Fig. 6). In con-trast to the core recovered at 70 m water depth (PALB 94-1E), these sedimentscontain OH-spheroidene and rhodopinal carotenoids that belong to the Spirilloxan-thin-series (e.g., Rhodopseudomonas and Thiodictyon respectively). The commoncarotenoids okenone and isorenieratene are also present at much higher concentra-tions than in core PALB 94-1E and cores from Lake Nemi. The profile of pha-ephorbides parallels that of cladocera (cf Manca et al. 1996, this volume).


Fig. 6. Lake Albano core PALB 94-6B. Selected preserved sedimentary algal and bacterialpigments and inferred primary productivity (for explanation see text). CONISS zones PI-PIXare shown. Units are per gram organic matter (g LOI-1). U = absorbance units. Ages given arecalibrated 14C years.

Anaerobic conditions were found in four main periods, as shown by the okenoneand rhodopinal profiles (Fig. 6: 720-615cm; 540-400 cm; 320-195 cm; 100-40 cm).Spheroidene without accompanying spheroidenone (a red carotenoid produced bythe oxidation of spherodene in the presence of trace amounts of O2) may be pre-sumed to signify a sustained period of meromixis in a shallow environment (Züllig1985).

The pigment data suggest that in addition to the bacteria, the chlorophyceae(lutein) and cyanobacteria (echinenone) are the most important taxa during periodsof high productivity, and at the base of the core (Fig. 6).

3.2.2. Diatoms

Diatom analysis on this core was undertaken for core correlation with PALB 94-1E, with counts of around 100 valves on 99 samples (Fig. 7). For this reason lim-nological interpretation can only be qualitative rather than quantitative. Formal dia-tom biozones were not constructed. Full details of the correlation with PALB 94-1Eare given in Juggins (1996).

Core 6B is entirely pre-Holocene, largely dominated by Cyclotella sp.1, with thepossible exception of the upper 20 cm or so, which may be a contemporary lag de-posit (Fig. 7).


Fig.

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but the proportion of non-plankton to plankton is greater in PALB 94-6B. Sedimentaccumulation is much higher in PALB 94-6B throughout the core, and revealsstructure within features that are expressed over a very narrow section of PALB 94-1E. Cyclotella pseudostelligera, for example, occurs over a ca 10 cm stratigraphicalsection in PALB 94-1E (from 793.5-801.5 cm), but is found over 75 cm in corePALB 94-6B (from 325-400 cm; Fig. 7).

Below 750 cm the proportion of planktonic taxa drops to 40% or less, and thedevelopment of Fragilaria brevistriata-complex and Amphora pediculus, in par-ticular, implies a very low lake level. The higher diatom non-plankton/plankton ratioin PALB 94-6B throughout can be expected from the shallower, more littoral loca-tion of the site. The relatively high proportions of epiphytic taxa such as Cocconeisspp and Gomphonema spp (especially in the uppermost 150-200 cm) may relate di-rectly to the development of macrophyte beds, or indirectly to lake level loweringbringing the source for these diatoms nearer the site. Alternatively, this could repre-sent transport of reworked sediments from littoral areas.

3.3. Lake Nemi: Core PNEM94-1B

Core PNEM 94-1B was mostly deposited in the Holocene, and correlates withHolocene sections of Lake Albano cores (see discussion below). The transition be-tween the Younger Dryas and Holocene is at ca 865 cm (Figs 8-10).


Dry mass peaks during the pre-Holocene, early and mid-Holocene (Fig. 8; zonesGI and GII). Carbonates are not very abundant (often below 10% d.m.) with maxi-mum values in zone GIII and the base of GI. Organic matter, organic carbon and or-ganic nitrogen profiles fluctuate greatly during the early-mid Holocene. The C:Nratios show variations indicating likely reduced inputs of organic matter from thecatchment in some periods.

Based on the pigment profiles (Fig. 9), five biozones can be separated during theHolocene. In general, the concentrations of pigments in this core are much highercompared to cores from Lake Albano. Pigments from sulphur photosynthetic bacteria(isorenieratene, okenone, rhodopinal, spheroidene and spheroidenone; Fig. 9), areabundant in the lower part of the core, especially in the laminated sediment layers(zones PI, PII, PIII and PV), indicating anoxic conditions. Zone PIV (4-3.5 kyr BP)is characterised by low pigment concentrations. Zone PV shows a slight increase inprimary productivity.

Higher and relatively constant values of CD:TC ratios in zone PV are the resultof less reducing conditions and relatively higher allochthonous input of organicmatter (Fig. 9). As in Lake Albano, primary productivity in the early Holocene isvery variable but greater compared to the late Holocene. Cyanobacterial pigmentscharacterise the early phase of the Holocene and lutein was also abundant (Fig. 9).Pigment concentrations rise again near the top of the core.


The pigment data suggest the sequence of Holocene algal development is verysimilar to that observed in Lake Albano (see above).

Fig. 8. Lake Nemi core PNEM 94-1B. Selected physical and geochemical parameters. Or-ganic matter = loss on ignition (LOI); BSiO2 = biogenic silica; % w.w. = % wet weight; %d.m. = % dry matter. CONISS zones GI-GIII are shown. Ages given are calibrated 14C years.

3.3.2. Diatoms

The diatom flora of Lake Nemi has been dominated by planktonic taxa through-out the core (Fig. 10), but five main zones can be recognised. At the bottom of thecore Cyclotella ocellata declines and small Stephanodiscus dominate the assem-blage, indicating a sequence of mesotrophic (Zone DI, dominated by S. minutulus),eutrophic (Zone DII, with S. hantzschii) and mesotrophic (Zone DIII, S. minutulus)conditions. Zone DIV (550-ca 200 cm) begins with a large isolated peak of Aula-coseira ambigua. Above this level, Stephanodiscus spp are less abundant, and Cy-clotella ocellata becomes important. This suggests that the lake became less pro-ductive and has remained so to the near present. At 300 cm Asterionella formosa andAulacoseira ambigua, which are typical of mesotrophic conditions, start to increase.At 200 cm (zone DV) Stephanodiscus minutulus increases, replaced at the top of thecore by S. hantzschii, representing a gradual increase in trophic state, which can belinked to anthropogenic impact.


Fig. 9. Lake Nemi core PNEM 94-1B. Selected preserved sedimentary algal and bacterialpigments and inferred primary productivity (for explanation see text). CONISS zones PI-PVare shown. Units are per gramme organic matter (g LOI-1). U = absorbance units; Ages givenare calibrated 14C years.

Zones DI, DII and DIII are very similar to contemporary Holocene Lake Albanodiatom assemblages while in the more recent past there is evidence of greater humanimpact at Lake Nemi.

Inferred total phosphorous (TP; Fig. 10) was reconstructed using the calibrationdata set of Wunsam & Schmidt (1995), as for Lake Albano (cf Fig. 3). Values,plotted on a logarithmic scale, appear to overestimate the highest TP episodes (from7-8 m for example) but can be used to identify trends in TP enrichment, and to dis-tinguish periods of lake enrichment. One reason for these large values may relate tothe occurrence within the fossil data of samples with much greater percentages ofhigh TP optima taxa relative to the surface sediment samples on which the transferfunction was calibrated. For example, many of the highest TP reconstructions aredriven by Stephanodiscus hantzschii, which has the highest optima (111.2 µgl-1 TP)but a maximum occurrence of only 7% within the Wunsam & Schmidt data setcompared to up to 80% in Lake Nemi sediments between 7-8 m (Fig. 10; cf Fig. 3).Inferred TP values may be overestimated in Lake Albano for similar reasons, but to asmaller extent.

Nonetheless, there does appear to be evidence that maximum lake TP values oc-curred in the early Holocene, as in Lake Albano, rising suddenly at around 8 m andfalling equally sharply around 7 m. By contrast, human activity in the catchment ap-pears to have less impact until the uppermost levels.


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4. PALAEOENVIRONMENTAL RECONSTRUCTION AT LAKE ALBANO &LAKE NEMI

4.1. Palaeoenvironmental reconstruction at Lake Albano: a synthesis

The close correspondence between the main diatom and pollen zones (Lowe etal. 1996, this volume) suggests that climate is likely to be an important determinantof lake water conditions (through various direct and indirect links).

The low number of chrysophyte cysts detected in Lake Albano sediment suggeststhat these remains are not very suitable for the reconstruction of the history of thislake. Nevertheless, the analysis of cyst assemblages provides an independent meansto test the trophic profile obtained by the use of diatom and pigment analysis.

Based on the integration of the information derived from the biological recordsexamined a preliminary palaeoenvironmental reconstruction is outlined below for themaster core PALB 94-1E, with reference to cores PALB 94-6B and the Holocenecore PALB 94-3A (not reported; Guilizzoni et al. 1996b).

FULL GLACIAL

• 1387.5-1349 cm: basal tephra layer (not an air-fall deposit; Calanchi et al. 1996,this volume).

• 1349-1322 cm: the lake is in its incipient stage, with biota colonising the newenvironment mainly using the aquatic vegetation as a substrate. Water level verylow; low productivity; oxidising conditions throughout water column and atsediment/water interface. Site PALB 6 (today at 30 m of water depth) is dry atthis time.

• 1322-995 cm: a relatively high (but not deep enough to cover site 6) and fluctu-ating water level (as suggested by higher proportions of non-planktonic taxa).Generally low productivity interrupted by sudden, relatively short-lived hightrophic episodes (peaks of pigments and the eutrophic diatom Stephanodiscusparvus replacing Cyclotella sp.1 completely in 2 phases of 20-40 cm duration;Devoy 1996). Occasionally reducing conditions at sediment-water interface; atthe end of the period lower water level. These productivity oscillations areprobably related to global climate changes (cf Oldfield 1996, this volume;Manca et al. 1996, this volume). Site PALB 6 most probably dry throughout thisperiod, but the base of PALB 94-6B may correlate to around 10 m in PALB 94-1E (see above; cf Oldfield 1996, this volume)

• 995-717 cm: generally low productivity in a rather deep and oxidising environ-ment at site 1. Maximum catchment erosion (Rolph et al. 1996, this volume).This section in PALB 94-1E covers almost the entire accumulation at site 6(represented by over 8 m in PALB 94-6B), which has a much faster accumulationrate. Core PALB 94-6B can therefore provide a better record in this section.

• PALB 94-6B: all proxy records reflect the more littoral and shallow location.The pigment and geochemical data in particular during this period suggest thelake experienced several large and rapid fluctuations in environmental conditions(cf Chondrogianni et al. 1996b, this volume). Core PALB 94-6B is capable of


furnishing a much clearer picture of these changes than PALB 94-1E due to acombination of factors. A higher sedimentation rate allows finer resolution eventsto be discerned, while the site may be sensitive to limnological change by virtueof its location within the lake system, both in terms of depth and proximity to theshore. Important limnological thresholds may be crossed at this site that are notpicked up in detail or in extent at other, deeper sites.

LATE GLACIAL

• 717-507 cm: Transition from late Glacial to Holocene. Productivity starts toincrease probably coinciding with the onset of stable meromictic condition (asindicated by preserved pigments from photosynthetic sulphur bacteria). Decreaseof Cyclotella sp. 1 and increase of Stephanodiscus minutulus and S. medius.Generally increasing abundance of biological remains, rising water temperatureand reduced erosion.

HOLOCENE

The entire Holocene may be largely covered by another Albano core, PALB 94-3A (data presented elsewhere; Guilizzoni et al. 1996b) taken in 120 m of waterwhich covers the period from the end of the late Glacial to the present. This core canbe correlated with Lake Nemi PALB 94-1B using pigment and pollen data and bycorrelation with the well-dated twin core PALB 94-3B from the same site (seeChondrogianni et al. 1996a, this volume). Proxies from core PALB 94-3A aresimilar to the records from Lake Nemi and appropriate Holocene sections of PALB94-1E (see below; Guilizzoni et al. 1996b) although sedimentation rate appearsmuch more variable at this deeper site.

• 507-377 cm: all the biological records point to maximum aquatic productivity(which fluctuates) and maximum reducing condition in the deeper water layer.Dominance of Stephanodiscus species. There is a recognised hiatus from ca 360-390 cm ( cf Oldfield 1996, this volume).

• 377-350 cm: the abrupt change registered in biological remains at ca 380-350 cm(ca 4 kyr BP) covers the period of the hiatus, and must include a mixture ofmaterial of indeterminate age from this period. Approximately 3400 years is rep-resented over this interval, from 4100 kyr BP at 355-360 cm, to 7500 kyr BP at390 cm (Oldfield 1996, this volume). For this reason proxy data cannot be relia-bly interpreted over this interval from this core. This interval can be filled bycore PALB 94-3A, however (or by the lake Nemi core).

• 350-147 cm: continuous sediment accumulation suddenly begins again during theearly Bronze age. Algal (and other) remains record the impact of human activityin the catchment, which appears to be increasingly important from this time.There is a sharp decline in productivity, a sudden increase in erosion and the endof reductive diagenesis. Pigment concentrations decrease abruptly, diatomassemblages shift from Stephanodiscus spp to Cyclotella spp and the strong andpersistent decrease in chrysophycean cyst concentrations (paralleled with


chironomids; Manca et al. 1996, this volume) is also interpreted as an indicationof human disturbance. An increase in non-tree pollen (e.g., Gramineae) providesevidence of forest clearance (Lowe et al. 1996, this volume). All of these changesare indicative of strong human impact at the base of this zone.

• From ca 147 cm (Roman Period) there is a higher level of productivity, withpeaks of photosynthetic sulphur bacteria and Stephanodiscus spp. Several strongfluctuations in these proxies are shown from here to the top of the core, reachingmaximum values in very recent times.Other signals of human impact on the lake ecosystem are recorded at ca 150 cm

and in the uppermost layers. During the past three centuries the sedimentation rate atsite PALB 94-1E, as estimated by varve counts, was 0.15 cm yr-1 (Lami et al. 1994).

4.2. Palaeoenvironmental reconstruction at Lake Nemi: a synthesis

Judging from the algal remain analyses five major periods/zones of different laketrophic state can be distinguished during the Holocene (cf Figs 8-10).

• 900-870 cm (pre-Holocene): At the base of the core, productivity (though higherthan Lake Albano) is low, relative to the rest of the sequence. The sedimentationrate at the beginning of the Holocene (0.025 cm yr-1) is very similar to cores fromLake Albano (e.g., PALB 94-3A; Guilizzoni et al. 1996b).

• 870-510 cm: the climatic amelioration of the early Holocene is reflected in bio-logical proxy records, with generally increasing but fluctuating trends inconcentration of pigments and diatom remains. A relatively high, but variable,trophic state is inferred during this interval from cyclically abundant pigments ofcyanobacteria, green algae and photosynthetic sulphur bacteria, blooms of theeutrophic diatom Stephanodiscus hantzschii. Pollen data imply the onset of an-thropogenic impact on the catchment during this section (Lowe et al. 1996, thisvolume). Sedimentation rate is estimated at 0.066 cm yr-1.

• 520-320 cm: a period of generally lower productivity, reduced concentrations ofpigments, increase of Graminaceae (Lowe et al. 1996, this volume). Human im-pact related to the Bronze Age may be responsible for the dramatic fall in pig-ment concentrations at ca 320 cm, while changes in zooplankton communitiessuggest catchment disturbance, such as deforestation (Manca et al. 1996, thisvolume). Lake levels may have been lower throughout the region; a mid-bronzeage villages were found at ca 14 m water depth at Lake Albano (Chiarucci 1994).This interval has an average sedimentation rate of 0.084 cm yr-1.

• 320-210 cm: this section is similar with generally low productivity, althoughcyanobacterial pigments and the total algal biomass increase. Pollen analysis im-plies a Roman age for the upper part of the zone and increasing cultivation in thecatchment from ca 200 cm (Lowe et al. 1996, this volume). Sedimentation ratesranged from 0.060 cm yr-1 to 0.120 cm yr-1.

• 210-0 cm: productivity increases (cf Fig. 9). Cyanobacterial pigment concentra-tions rise, Stephanodiscus spp develop, and in recent times diagnostic changes infaunal communities (Manca et al. 1996, this volume). Sedimentation rates aver-


age 0.100 cm yr-1, increasing to 0.38-0.60 cm yr-1 during this century (ages cal-culated by varve counting; Masaferro et al. 1993; Lami et al. 1994).

4.3. A palaeolimnological comparison of Lake Albano and Lake Nemi

Similarities in lake history between these two sites can be expected from theircommon origin (hydrogeochemistry, catchment geology) and regional context(climatic influences), and broadly speaking these are reflected in the algal proxy re-cords that have been studied. There are some notable differences, which may stemfrom their distinct bathymetry, catchment size and topography, and history of humanactivity at each site. An objective comparison is more difficult in practice. Firstly,only one core from Lake Nemi has been analysed, while the several cores taken andexamined from Albano have served to show that each site has recorded LakeAlbano's limnology from a different aspect. Secondly, the resolution of analysescarried out on each core has differed in terms of sampling interval and for diatomanalysis, the precision of the count (cores PALB 94-3A and PALB 94-6B werecounted to a lower total per sample for correlation with PALB 94-1E). Importantly,multiple coring has shown that none of the cores from Lake Albano contains con-tinuous sedimentation throughout the lake's history. Hiatuses are difficult to detect ina single core which spans the entire lake history (but does not include it all).

Most records in the Lake Nemi core (PNEM 94-1B) show similar trends to corescovering the same period from Lake Albano (PALB 94-1E and the Holocene coresfrom Station 3). Accumulation rates are similar or slightly lower in PALB 94-1Efrom those periods when sediment was accumulating (507 cm over ca 6,600 years inPALB 94-1E, compared to 870 cm in PNEM 94-1B over the entire Holocene), andtrends in most algal remains correlate well between lakes. It is this coherence, in fact,that has allowed the chronology of PNEM 94-1B to be securely tied to well-datedLake Albano cores from station 3 (Juggins 1996; Rolph et al. 1996, this volume) andthe mis-match with pollen data that firmly established the hiatus in PALB 94-1E(Lowe et al. 1996, this volume; Oldfield 1996, this volume).

Palaeoproductivity inferred from preserved pigments and TP inferred from dia-tom assemblages in core PNEM 94-1B agree well with similar reconstructions fromPALB 94-1E (Figs 2 and 3) and PALB 94-3A (Guilizzoni et al. 1996b). Pigmentconcentrations are much higher in PNEM 94-1B compared with PALB 94-1E (2-5times greater) and PALB 94-3A, notably in the basal zone of PNEM 94-1B. Thereare differences in the diatom records, for example, most notably the appearance ofAulacoseira ambigua and A. granulata in PNEM 94-1B from about 525 cm to thesurface. Conversely Stephanodiscus parvus seems confined to Lake Albano, andappears in both PALB 94-1E and PALB 94-3A; while S. hantzschii is much moreabundant in Lake Nemi from about 7-8 m (with higher TP values inferred as a re-sult). Primary productivity inferred from preserved pigments (although not diatoms)suggest that although trophic trends are similar in both lakes, levels are higher in theupper sediments of Lake Nemi than contemporaneous Lake Albano sediments(compare Figs 2, 9; and Figs 3, 10). This may imply less anthropogenic nutrient en-richment of Lake Albano in the late Holocene, although pollen evidence suggests


considerable catchment disturbance in both lakes (Lowe et al. 1996, this volume)and there is archaeological evidence of Roman settlement at both lakes (Chiarucci1978).

5. CONCLUSIONS

5.1. The nature of variability in late Quaternary lacustrine sedimentary records

There appear to have been several sudden warm events on a century time scale inthe glacial period, judging from the Lake Albano cores (Figs 1-3; Manca et al. 1996,this volume; Devoy 1996). At these times, some algal records display behaviourmore typical of the full Holocene (such as the diatom flora). These are phases offairly high lake productivity with a seasonally anoxic water column separated bylong oligotrophic cold periods of low water level (Figs 2, 3). The main oscillations incore PALB 94-1E have an estimated cycle duration of ca 200-500 years for theperiod ca 23 kyr-30 kyr BP (cycles of 90-650 years in core PALB 94-6B, for theinterval 23 kyr-17 kyr BP). In many records higher-frequency fluctuations can bediscerned within these major cycles, implying they possess complex structure.

The two most likely categories of hypotheses for these sharp productivity oscil-lations are endogenetic (e.g., geothermal activity) and exogenetic (regional climaticwarming). At the present it is not possible to rigorously exclude an endogenetic ex-planation, although the balance of probabilities is perhaps against such an interpre-tation (Oldfield 1996, this volume).

In contrast, similar warm excursions within the full glacial exist from otherstudies in the region and have been observed in the pollen profiles of a core fromLake Albano (Lowe et al. 1996, this volume), from a more southerly site, LagoGrande di Monticchio (Huntley et al. 1996; Watts et al. 1996a, 1996b), and from astudy on mammal changes in southern Italy (Abbazzi et al. 1996).

Moreover, according to Chondrogianni et al. (1996b, this volume), Manca et al.(1996, this volume), Oldfield (1996, this volume) and Huntley et al. (1996), suchrecords (although dating of pre-Holocene sections of Lake Albano cores is not pre-cise enough to correlate such events with sufficient accuracy) can be associated withthe palaeoenvironmental records from the North Atlantic and Greenland (Dansgaardet al., 1984; Bond et al. 1993). Further high resolution work in dated pre-Holocenesediment sequences in the region is needed to confirm that these climatic oscillationsare widespread, coeval events not a seismic-volcanic origin.

Holocene palaeoenvironments can be reconstructed with greater temporal andspatial resolution. Dating is much more reliable during the Holocene in sequencesfrom both Lake Albano and Lake Nemi. High productivity levels appear during thefirst half of the Holocene, a period in which inferred primary productivity and totalwater phosphorus concentrations reached values found in very eutrophic lakes today(Figs 2, 3, 9, 10). Oscillations in lake productivity can be compared with similarmajor vegetational changes reported from the Valle di Castiglione and Lagaccione(central Italy, Alessio et al. 1986, Magri & Follieri 1989) and Lago Grande di Mon-ticchio (Huntley et al. 1996).


There is clear evidence from many independent records of human impact duringthe Mesolithic, the Bronze age and Roman times in both Lake Albano and LakeNemi. There is some suggestion both from the pigment and diatom data that inferredproductivity fell from extremely high levels at the start of the Holocene at the onsetof human activity (Figs 2, 3, 9, 10), subsequently rising to high, stable levels wellbefore the Roman period. From this time throughout the historical period there islittle evidence of anthropogenic enrichment until modern times. Human impacts,outside the modern era, appear within the range of natural variation of these systems(for example, during the early Holocene). This points both to the high intrinsic vari-ability of crater lake systems and the often extreme effects of the most recent humanactivity which alone may be unprecedented in the lakes' Holocene histories.

Algal remains provide an internally consistent record of ecosystem response toseveral well-defined natural and anthropogenic events, reaffirming the close inter-relationship within this trophic level. It is clear from the summary figures presentedhere and elsewhere in this volume (e.g., Manca et al. 1996; Lowe et al. 1996; Old-field 1996) that there is remarkable coherence across a diverse range of biotic andabiotic proxy records recorded in the sediments, which demonstrates the inter-de-pendence of the entire lake-catchment system as it responds to environmentalchange. A multi-proxy approach integrates these apparently distinct signals and re-veals their linkages, often in previously unsuspected ways, and provides the meansby which hypotheses about environmental change can be tested. Contradictory sig-nals can be resolved by a jury of other proxies.

Interpretations based on one or a few signals inevitably produces biased andopen-ended hypotheses especially if the signal is discontinuous for part of the record(such as the no-analogue phase of the diatom record, periods of poor pigment pres-ervation or dissolution events in the magnetic signal; Rolph et al. 1996, this volume).A multi-proxy approach minimises the chances of such data gaps and indeterminacy.

ACKNOWLEDGMENTS

We are grateful to Rag. R. Ferro, Director of the Centro Federale C.O.N.I.,Castelgandolfo (Roma), the Federazione Italiana Canoa, Roma, Mr. O. Catarci,Nemi (Roma), and the Federazione Italiana Sci Nautico, Castelgandolfo (Roma), fortheir advice and logistical support during field work. Special thanks are due to ourPALICLAS colleagues L. Langone, CNR-IGM, Bologna, F. Lucchini, Dept. EarthSciences, Univ. Bologna, and C. Chondrogianni and D. Ariztegui, ETH-Zentrum,Zürich, who provided us with the data on carbon, nitrogen, biogenic silica, andsediment dry matter. Thanks also to Prof. C. Andreis of University of Milan for theidentification of bryophytes in some Pleistocene sediments. Dr. S. Wunsam kindlyprovided her Alpine diatom data set, and was very helpful in discussions of diatomtaxonomy. Prof. N.J. Anderson and Dr. H. Håkansson also provided guidance intaxonomic matters. Work supported by the EU, contract No EV5V-CT 93-0267,DG XII-FZPA.


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