Phosphorus loadings to lakes and some of their responses. Part 2. Regression models of summer phytoplankton standing crops, winter total P, and transparency of New York lakes with known phosphorus loadings’ R. T. Oglesby and W. R. Schaffner Department of Natural Resources, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853 Abstract An interrelated series of regression equations is derived to define the response of lake ecosystems to mixed zone phosphorus loadings in terms of simple and readily determinable parameters. Data used were collected over 14 yr for 16 lakes in central New York State characterized by a wide variety of morphometries, hydrologies, and phosphorus loadings. The regressions describing the dependence of summer phytoplankton standing crop and winter total P concentration on loading and of standing crop on total P were linear. Those characterizing water transparency as a function of standing crop and winter total P were parabolic. All regressions showed high correlation coefficients. The overall model composed of these regressions establishes a quantitative basis for the concept that phosphorus supplied to lakes exerts a uniquely controlling role on phytoplankton standing crop. The regressions may be used for trophic state description and in the develop- ment of management strategies for lakes. Schaffner and Oglesby (1978) proposed a new way of expressing phosphorus loading to lakes in which a composite species of phosphorus, intended to repre- sent that of biological significance, is added on an annual basis to a volume equivalent to that of the summer mixed zone. Here we examine some of the re- sponses of lakes to mixed zone loading (A’,,) as an interrelated series of sim- ple regression models. The parameters selected to represent the responses of lakes were chosen on the basis of their ecological significance, of the readiness with which reliable infor- mation on them could be obtained from the literature or by actual measurement, and of their practical value. The concen- tration of phytoplankton pigments in the upper part of the water column during summer, winter phosphorus concentra- tion representative of the entire water column, and water transparency during summer met all of these requirements to some degree. Among other possible mea- l This research was supported by funds from the Rockefeller Foundation and the Office of Water Re- sources Technology (project B-038-NY). sures that might have been selected are primary productivity and hypolimnetic dissolved oxygen. Data on the former are not numerous, especially over a season or full year. Dissolved oxygen concentra- tions in the hypolimnion are of value in following changes in a single lake but their usefulness for comparing different systems is complicated by the influence of lake morphology. In assessing the response of lakes to phosphorus loading we needed compa- rable data on both the response parame- ters and soluble phosphorus supply for either a group of lakes or a single lake for which loading had changed over time. We could not find in the literature the kind of reliable information, capable of being fitted into a uniform format, that we needed. We therefore decided to collect data on the responses of those lakes for which loading was to be calculated. Be- cause interrelations between the various response parameters were to be ex- amined, we also collected information to describe several other lakes for which loading estimates could not be calcu- lated. Our success was due in no small part to 1,IMNOLOGY AND OCEANOGRAPHY 135 JANUARY 1978, V. 23(l)
Transcript
Phosphorus loadings to lakes and some of their responses. Part 2. Regression models of summer phytoplankton standing crops, winter total P, and transparency of New York lakes with known phosphorus loadings’
R. T. Oglesby and W. R. Schaffner Department of Natural Resources, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853
Abstract
An interrelated series of regression equations is derived to define the response of lake ecosystems to mixed zone phosphorus loadings in terms of simple and readily determinable parameters. Data used were collected over 14 yr for 16 lakes in central New York State characterized by a wide variety of morphometries, hydrologies, and phosphorus loadings.
The regressions describing the dependence of summer phytoplankton standing crop and winter total P concentration on loading and of standing crop on total P were linear. Those characterizing water transparency as a function of standing crop and winter total P were parabolic. All regressions showed high correlation coefficients.
The overall model composed of these regressions establishes a quantitative basis for the concept that phosphorus supplied to lakes exerts a uniquely controlling role on phytoplankton standing crop. The regressions may be used for trophic state description and in the develop- ment of management strategies for lakes.
Schaffner and Oglesby (1978) proposed a new way of expressing phosphorus loading to lakes in which a composite species of phosphorus, intended to repre- sent that of biological significance, is added on an annual basis to a volume equivalent to that of the summer mixed zone. Here we examine some of the re- sponses of lakes to mixed zone loading (A’,,) as an interrelated series of sim- ple regression models.
The parameters selected to represent the responses of lakes were chosen on the basis of their ecological significance, of the readiness with which reliable infor- mation on them could be obtained from the literature or by actual measurement, and of their practical value. The concen- tration of phytoplankton pigments in the upper part of the water column during summer, winter phosphorus concentra- tion representative of the entire water column, and water transparency during summer met all of these requirements to some degree. Among other possible mea-
l This research was supported by funds from the Rockefeller Foundation and the Office of Water Re- sources Technology (project B-038-NY).
sures that might have been selected are primary productivity and hypolimnetic dissolved oxygen. Data on the former are not numerous, especially over a season or full year. Dissolved oxygen concentra- tions in the hypolimnion are of value in following changes in a single lake but their usefulness for comparing different systems is complicated by the influence of lake morphology.
In assessing the response of lakes to phosphorus loading we needed compa- rable data on both the response parame- ters and soluble phosphorus supply for either a group of lakes or a single lake for which loading had changed over time. We could not find in the literature the kind of reliable information, capable of being fitted into a uniform format, that we needed. We therefore decided to collect data on the responses of those lakes for which loading was to be calculated. Be- cause interrelations between the various response parameters were to be ex- amined, we also collected information to describe several other lakes for which loading estimates could not be calcu- lated.
Our success was due in no small part to 1,IMNOLOGY AND OCEANOGRAPHY 135 JANUARY 1978, V. 23(l)
136 Oglesby and Schaffner
having a set of comparable data for a large number of lakes. For this we are indebted to P. Godfrey, J. McKenna, M. Ginsparg, and especially to E. Mills and D. Cham- berlain. P. J. Dillon’s helpful comments on the manuscript are also acknowl- edged.
Limnology of study lakes
Sixteen lakes in central New York State, representing wide ranges of proba- ble nutrient input, size, depth, and hy- drologic regime were selected. Eleven of these were the Finger Lakes, about which Birge and Juday (1914, p. 537) wrote “It is probable that there is no group of lakes in the world which offers to the limnologist such opportunities for working out the problems of his science.” Waneta and Lamoka are small, relatively shallow bodies of water to the south of Keuka; Oneida is also shallow but with a large, windswept surface; Otsego (James Fennimore Cooper’s “Glimmerglass”) is similar in morphology to the Finger Lakes but lies some 120 km east of Otisco; and Canadarago, of moderate depth, is of special interest because it has been the site of an intensive program to control phosphorus input through tertiary sewage treatment (Hetling and Sykes 1971). The pertinent morphometric pa- rameters for 13 of the lakes are given by Schaffner and Oglesby (1978: table 3). In addition, Lamoka, Otsego, and Waneta have mean depths of 5.0, 12.6, and 3.5 m; surface areas of 2.3,51.0, and 3.2 km2; and volumes of 11.6,210.4, and 10.9 x lo6 m3. Mean hydraulic retention times have not been estimated for these three lakes. Mean and maximum depths vary be- tween the lakes by more than a factor of 10, surface areas by as much as two, and volumes by over three orders of mag- nitude. The longest mean hydraulic re- tention time (Seneca) is 23 times greater than the shortest, for Honeoye. Popula- tion and land use in the basins were de- scribed by Schaffner and Oglesby (1978: tables 1 and 2).
Of the 16 lakes, Schaffner and Oglesby (1978) calculated loadings for 13; five
cc onesus, Hemlock, Cayuga, Owasco, and Skaneateles) were studied inten- sively for 2 or more years (Oglesby et al. 1975; Mills 1975; Chamberlain 1975; Schaffner and Oglesby in press; Godfrey 1977); for the remaining 11 we made dur- ing 1973 at least one determination of winter total phosphorus (samples col- lected 23-30 April) and two of summer chlorophyll and transparency (17-18 July and 28-31 August), along with other lim- nological measurements. Additional data for 1968-1969 (G reeson 1971) and for 1975-1976 (Mills unpublished) were available for Oneida Lake.
All samples were taken with PVC Van Dorn bottles and transferred to phosphorus-free polyethylene contain- ers. Chlorophyll concentrations express the average of samples taken at 0, 5, and 10 m through 1972 with the 2-m depth added after that. Transparency was mea- sured with a 2O-cm-diameter Secchi disk (Wildlife Supply) divided into alternating black and white quadrants. Winter total P concentrations represent average values for samples collected at the surface and just off the bottom; occasional samples at intermediate depths indicated that sur- face and bottom values were sufficient.
Pigment analyses were always begun and phosphorus analyses completed on the day of sampling. Chlorophyll a plus pheopigment was determined, after filt- ration onto a Reeve Angel 984 AH glass- fiber filter, according to the method of Strickland and Parsons (1972) and con- centrations calculated by the equations of Lorenzcn (1967). Measurements were made in a I-cm cell with a spec- trophotometer (Turner 330). Methods for phosphorus analysis were given by Schaffner and Oglesby (1978); absor- bance was measured in a lo-cm cell in a spectrophotometer (Beckman DU).
Those lakes that have been com- prehensively studied exhibit seasonal changes typical of temperate latitudes with summer minima of soluble reactive phosphorus and nitrate nitrogen and spring and fall minima of reactive silicon. The pattern is exemplified by the values shown in Table 1 for four of the Finger
Phosphorus loadings to lakes. 2. 137
lakes. There are large differences in NOB--N and reactive silicon between lakes. The former decreases markedly in those Finger Lakes west of Keuka. Sum- mer nitrate levels in Oneida (Greeson 1971), Canadarago, and Waneta also ap- pear to be low. Limited data indicate that of the Finger Lakes the two largest, Seneca and Cayuga, have the lowest epilimnetic reactive silicon concentra- tions.
We find highly persuasive the pub- lished evidence and arguments that phosphorus generally controls phyto- plankton production and standing crops in temperate lakes. Nevertheless, nitro- gen and several trace elements may be present in small enough quantities to limit phytoplankton growth at times in one or more of the lakes studied. Mills and Og- lesby (1971) found Co, Cu, and Zn levels in Cayuga that are among the lowest so far reported for any lake in the world. Iron and manganese are present at very low concentrations for at least part of the year in all of the Finger Lakes (Schaffner and Oglesby in press) and in Oneida (Greeson 1971). Nitrate-N may be de- pleted to concentrations so low as to be undetectable (sensitivity of analytical method about 10 mg rno3) at times during summer (Oglesby et al. 1975; Stewart and Markello 1974). However neither trace metals nor available nitrogen apparently exert a major control over phytoplankton standing crops in the five Finger Lakes that have been extensively studied. N fix- ation (acetylene reduction method) was detected at times in Conesus Lake and increased heterocyst formation by algae such as Aphanixomenon flos-aquae was observed during periods when NO,--N concentrations were very low, For exam- plc, during fall 1972, there was a major bloom (Chl a + pheopigment reached a level of about 30 mg rnM3 throughout the water column) of A. flos-aquae. Nitrate concentration was initially low and quickly reached levels of undetectability. Aphanixomenon was soon largely re- placed by Melosira, but during this change NO,--N showed little or no in- crease. Increases in phytoplankton stand-
Table 1. Seasonal average concentrations (mg m-3) for 1972-1973, integrated over depth and time, of soluble reactive phosphorus (SRP), nitrate nitro- gen (NO,--N) and soluble reactive silicon (SRS) in the upper 10 m of the water column (data from Mills 1975).
*CO = Conesus, HE = Hemlock, OW = Owasco, SK = Skaneatelcs.
ing crops occurred even with one or more trace metals present at extremely low concentration.
On the other hand, there is substantial evidence to suggest that phosphorus exerts a major controlling influence over phytoplankton production during sum- mer. Patterns of seasonal change with very low (ca. < l-2 mg mV3) soluble re- active phosphorus levels occurring at in- tervals during summer are themselves strong presumptive evidence of limita- tion by this element. Other indications come from an extensive set of bioassy studies (Peterson et al. 1974) on natural communities of phytoplankton from Cayuga Lake and research on in vivo al- kaline phosphatase activity as a function of soluble reactive phosphorus levels, also for Cayuga (Griffin 1974). Increases in alkaline phosphatase activity in net plankton during periods when soluble P levels were low and decreases when phos- phorus concentrations increased have also been found for summer communi- ties in Conesus, Hemlock, Owasco, and Skaneateles lakes.
Among the limnological factors indica- tive of lake trophic state, other than nu-
138 Oglesby and Schaffner
25
t 9 .0.24x - 4.05 r2 = 0.78 /
01 1 I I L 1 0 20 40 60 80 100 120
f-imp (mg.m’3.yr-1)
Fin. 1. Regression of mean summer Chl a + pheopigment for upper 10 m of water column (CHL,) on annual mixed zone loading of biologi- cally available phosphorus (A’,,).
trient supply and phytoplankton standing crops which are discussed below, are dis- solved oxygen in the hypolimnion and the composition and rates of productivity of the phytoplankton. In general, the larger and deeper lakes included in this study have hypolimnetic dissolved oxy- gen minima above 6 mg liter-l. Con- centrations may be depressed to about 2 mg liter-l in Hemlock, Otisco, and Canadice. In some years the deepest waters of Conesus, and possibly others of comparable or smaller size and depth, become anaerobic. Among the remain- ing lakes, Waneta and Lamoka had hypolimnetic minima of l-3 mg liter-’ in August 1973 and Oneida exhibits trans- ient anaerobic conditions in its deeper waters during periods of wind calm. Dis- solved oxygen in Canadarago Lake was not measured. Comparing historical in- formation for these lakes, especially that of Birge and Juday (1914, 1921), with that of recent years we conclude that hypolimnetic oxygen depression is more a function of lake morphology than of dif- ferences in productivity between lakes or of changes in productivity for any single lake over time.
About 400 taxa of phytoplankton were identified during the 1972-1973 Finger Lake studies (Mills 1975; Godfrey 1977). In very broad terms, we can say that the shallower lakes, such as Conesus, Hon- eoye, and Oneida, are more prone to have summer blue-green populations reaching nuisance levels. Primary productivity data are available for Cayuga only (1957-1958: Howard 1963; 196&1970: Peterson 1971.) However, even this lim- ited information shows the differences possible between years having presuma- bly similar loadings. Howard found that productivity rates, calculated on an areal basis, differed by a factor of about five between the 2 years, a difference that probably is near the extreme range of short term variation for this lake.
Phosphorus loading and phytoplankton standing crop
Schaffner and Oglesby (1978) gave methods for calculating the annual sup- ply of biologically available phosphorus (BAP) to lakes, together with values de- termined for 13 New York lakes. The biological response of receiving systems to this input was measured in terms of phytoplankton standing crop, as esti- mated by the mean concentration of Chl a + pheopigment (CHL,) in the upper 10 m of the water column during June, July, and August. For the New York lakes that have been definitively studied, the June- August standing crops gave the same relative relationships as did those for the entire year. This provided a ratio- nale for using the summer period and permitted the inclusion of lakes that had not been sampled on an annual basis. We also chose this particular measure for practical reasons. In central New York, June through August is the major recre- ation season-the time when the public is concerned about the visual aspects of water quality. The 10-m water column was chosen because many of the pub- lished data are expressed as concentra- tions at O-IO-m depths, and this gave a relation that could be tested for other lakes. Ten meters is also a reasonable ap- proximation of the depth of the thermo-
Phosphorus loadings to lakes. 2. 139
Table 2. Summer pigment (CHL,) and winter phosphorus (TP,) concentrations for New York lakes.
*Each sample represents an average of concentrations measured at the surface, 5-, and 10--m depths. A 2-m sampling depth was added in some instances duriny 1973.
?TP, values represent vertical profile means.
sU.S. Environmental Protection Agency (19742).
§Hamilton and Barlow (1966).
11 U.S. Environmental Protection Agency (1974_b).
**B. J. Peterson and J. P. Barlow (personal communication).
.I--bSelected from Greeson (1971).
$tE. L. Mills (personal communication).
Cline, and a significant part of the euphotic zone for moderate-sized lakes in temper-
There is no reason to think that A’,,,
ate latitudes. The pigment data are pie- as it affected phosphorus availability dur-
sented in Table 2. Standard errors of the ing the summer growing season, varied
means were calculated for values repre- appreciably during 1971-1973. We there- fore assume the scatter about the re-
sented by more than four samplings. The relation between phosphorus loading to
gression line is a function of changes in
the mixed zone (A’,,) and standing crop standing crop in response to factors other
(CHL,) is shown in Fig. 1. than phosphorus loading, such as avail- able light, degree of mixing, differences
140 Oglesby and Schaffner
PO -
TP, ( mg mm3 )
Fig. 2. Regression of mean summer Chl a + pheopigment for upper 10 m of water column (CHL,) on winter concentration of total phosphorus (TP,) for deeper New York lakes.
in the rates at which phytoplankton was grazed, and to inaccuracies in estimating &llP and CHL,.
Howard’s (1963) study of primary pro- ductivity during 1957 and 1958 in Cayuga Lake did not include CHL, measure- ments. However, if the 1972-1974 chloro- phyll averages for Cayuga arc used as reference points, and if productivity is assumed proportional to CHLs, a varia- tion of the magnitude Howard described would have produced mean summer chlorophyll concentrations ranging from about 5.5-11 mg rnm3. In other words, one piece of evidence indicates that factors other than phosphorus loading could have produced variations in the average 1972-1974 value (8.7 mg mw3) of about 25-35%.
Initially, the specific loadings (L,,) as calculated by Vollenweider (1968) were used as the independent variable in the regression (Oglesby and Schaffner 1975). However, Alrnp p roduced a much better correlation.
Winter total phosphorus and phytoplankton standing crop
The concentration of total phosphorus in the water column of a lake during win- ter (TP,) has been used as an index of the phosphorus that will be available to sup- port phytoplankton growth the following summer. Sawyer (1947) used this in a rather general (and often misquoted) way
J 10
TP, (mg KS)
Fig. 3. Regression of CHL, on TP, for deeper New York lakes extrapolated to include range of values for Lake Washington (1957-1971). Redrawn from Oglesby and Schaffner (1975).
to connect trophic status with nutrient levels in the Madison (Wisconsin) lakes. Sakamoto (1967) tested the applicability of the concept to a large number of lakes, and his work was expanded on by Dil- lon and Rigler (1974a) with data for North America and Europe. Both estab- lished relationships that were linear-but only on a log-log plot-thus imposing very large confidence intervals, espe- cially at higher TP, and chlorophyll con- centrations, on their regressions.
We tested the dependence of mean summer chlorophyll a plus pheopigmcnt in the upper 10 m of the water column on TP, for the 16 New York lakes (Table 2) and got the regression, linear on an arithmetic scale:
Y = 0.574X - 2.90; r2 = 0.82
where Y is CHL, and X is TP,, both in mg mw3.
P. J. Dillon (pers. comm.) has pointed out the absence of very low values in this regression. His work suggested a second relation, of decreased slope, is charac- teristic of lakes with very low TP, con- centrations. It is also logical to suppose that at very high values of TP, (or of A’,,) slope will decrease as phosphorus ceases to be the factor controlling phyto- plankton production.
The unique historical data available for
Phosphorus loadings to lakes. 2. 141
Lake Washington (Edmondson 1972) al- lowed us to test the applicability of the TP,-CHL, relation of the New York lakes for another body of water, one. in which phosphorus inputs increased sub- stantially (1957-1963) and then decreased as sewage effluents were diverted (1963- 1968). For maximum comparability we calculated a regression for the deeper Finger Lakes plus Otsego and Canadarago only (Fig. 2); this was then extrapolated to encompass the range of values for Lake Washington (Fig. 3). The shaded area in Fig. 3 represents 95% confidence intervals about the mean values for the New York lakes. The dashed lines on either side of the regression indicate 95% confidence intervals for predicted values. The points representing Lake Washington are designated by year and connected by arrows. The extrapolated regression for our data rather accurately describes a series of events in another lake -one that is deep and cold as are the Finger Lakes but that is markedly differ- ent in some other properties (e.g. total dissolved solids, climate).
It should bc stressed that winter total phosphorus is considered as an index of the annual phosphorus pool rather than as a measure of the actual quantities available to phytoplankton during the fol- lowing growing season.
Phosphorus loading and winter total phosphorus
Given the previously established rela- tions of TP, and A’,, to CHLB, we would expect a correlation between the first two of these parameters; this is indeed the case (Fig. 4). The regression thus completes a rational link between AlrnP and CHL, and is itself an expression of the relation between phosphorus in the water column and that supplied an- nually to the lake from outside sources. As with the CHL&‘,, regression, the high degree of correlation indicates that effects of depth, and indirectly of hy- draulic retention time, are adequately ac- counted for by the way in which phos- phorus supply to the New York lakes is calculated. The intercept of this regres-
0 1 I I 1 1 I 0 20 40 60 80 100 120
4np (mg m-“.yr-1)
Fig. 4. Rcgrcssion of winter total phosphorus (TP,) on annual mixed zone loading of biologically available phosphorus (A’,&.
sion at the intersection of the X and Y axes suggests that the choice of a I-yr interval to represent the annual re- plenishment cycle is appropriate for de- scribing the phosphorus dynamics of these systems.
Summer phytoplankton stunding crop and water trunsparency
Several limnologists (e.g. Edmondson 1972; Bachmann and Jones 1974; Carlson 1977) have shown a correlation between Secchi disk transparency (SDT) and chlo- rophyll concentration. With values influ- enced by nonchlorophyllous suspended material such as allochthonous sediments excluded, Secchi disk transparencies averaged for the summer season (SDT,) can be used to indicate trophic state for lakes for which only the simplest data are available or can be collected. At very high chlorophyll concentrations there will be relatively little change in trans- parency as phytoplankton standing crop is changed (Edmondson 1972).
The following relation showing the dependence of SDT, on CHL, was de- rived from about 400 observations on New York lakes, and an almost equal number from Lake Washington (Ed- mondson 1972) added to define the high
142 Oglesby and Schaffner
(3 EXTERNAL
%Y
Fig. 5. Interrelations of regressions expressing some responses of New York lakes to annual mixed zone loading of biologically available phosphorus.
chlorophyll-low the curve.
transparency portion of
loglo Y = 0.961 - 0.606 log,, X; r2 = 0.85
where Y is SDT, (average summer Secchi disk transparency for a given lake during a particular year) and X is CHL,. The logarithmic function expressing depen- dence of transparency on chlorophyll concentration is what might be expected according to the Beer-Lambert law if the primary effect of phytoplankton on trans- parency were absorption of light. A simi- lar relation of SDT, to TP, is to be expected in view of the high linear cor- relation of the latter with CHL,.
Winter total phosphorus water transparency
and summer
An internal check on the consistency of the TP,-CHL,-SDT, chain of relations is provided by examining the dependence of transparency on winter concentrations of total phosphorus. For the New York lakes studied this was
loglo Y = 1.36 - 0.764 loglo X; r2 = 0.81
where Y is SDT, and X is TP,.
Discussion
The regression equations developed in this study can be connected in a logical sequence (Fig. 5) if the following as- sumptions are made: that the concen- tration of phytoplankton pigments in summer is a function of phosphorus, ex- pressed as a biologically available com- posite of soluble and potentially soluble forms, supplied from external sources annually and of lake mixing depth; that winter total P depends on the annual supply to the lake and serves as an index of the phosphorus pool available to phytoplankton during the following grow- ing season; and that transparency, as measured by the Secchi disk, is a func- tion of phytoplankton abundance and this is also reflected in an indirect relation to TP,. The high correlation coefficients indicate that these are valid working pos- tulates for the New York lakes studied.
Both depth (Thienemann 1927; Raw- son 1952, 1955; Vollenweider 1968) and hydraulic retention time (Vollenweider and Dillon 1974; Dillon and Rigler 1974b; Dillon 1975) have been proposed as important determinants of lake trophic state. We believe that our method of cal- culating loading provides not only a ra- tional but also a simple way of dealing with the depth factor. With mean hydrau- lic retention times (T,) for the lakes rang- ing from 0.6-18 yr, the high correlation for our CHLsR’,, regression indicates that water renewal time is probably ac- counted for adequately. This is logical since T, is related to the volume of a lake and this in turn is a function of depth. However, caution seems advisable in ex- trapolating this conclusion to lakes with retention times shorter than about 0.5 yr, since at some point washout of nutrients and phytoplankton will exceed their rate of replenishment. Dillon (1975) found that this was apparently the case for a lake with T, of 0.21-0.22 yr.
Our general approach to the response
Phosphorus loadings to lakes. 2. 143
of lakes to nutrient loading in some ways resembles that used by Shannon and Brezonik (1972) in their study of 55 shal- low Florida lakes. They combined seven environmental variables by principal components analysis to form a trophic state index (TSI) that correlated well with volumetric loadings of both phosphorus and nitrogen. Of the parameters (primary productivity, chlorophyll a, total P, total organic N, Secchi disk transparency, spe- cific conductivity, and the ratio of Na + K to Ca + Mg) used to compute their TSI, we found chlorophyll a, total P (if winter concentrations are used), and SDT (if ex- pressed as a log function) applicable to New York lakes. However, we found no significant correlation between conduc- tivity or the cation ratio and phosphorus loadings.
We did not attempt to follow the Zead of Shannon and Brezonik and develop a new index for New York lakes. Although reducing lake response to a single vari- able has the advantage of simplicity of presentation, this is offset, especially where a group of new relations is pro- posed, by the complexity of calculation and interpretation. We believe that by leaving the response parameters and in- terrelations separate we will encourage limnologists to assess their applicability to other lakes, and, that even at this stage of development, the individual relations may be of use in making management de- cisions.
Whether the relations established here for a group of New York lakes can bc cx- trapolated to other situations remains un- tested. The following caveats should be added to that above for water retention time. At very high loadings phosphorus may no longer bc the factor controlling phytoplankton production. Preliminary indications from a survey of the literature on other lakes throughout the world are that in humid, temperate climates the New York lakes regressions for both CHL, and TP, hold for lakes with mixed depth loadings of soluble phosphorus up to 200-300 mg rnA3 yr-l. Deviations from the CIIL,A’,, and CHL,TP, regressions may occur where turbidity due to allo-
chthonous materials or resuspended bot- tom sediments, or both, persists during the growing season or where humic sub- stances limit avialable light. SDT, would also cease to be a meaningful function of CHL, under such circumstances. For lakes with phytoplankton peaks at ti mes other than midsummer (e.g. many eutro- phic lakes in Great Britain and Europe), adjustments will have to be made in the period for which CHL, and SDT, are cal- culated. In situations where nutrient in- puts undergo sudden, large changes there may be a period of disequilibrium between phosphorus and standing crop that will cause deviations from our re- gressions. However, the response of Lake Washington to changes in phosphorus input suggests that equilibria may be es- tablished rapidly even in lakes with rela- tively long water retention times.
We also expect variations from our re- gressions simply because they represent highly generalized relations that do not take into account many biotic and abiotic factors that exert substantial short term influences on phytoplankton standing crops and on the internal recycling of phosphorus. These influences are bound to produce variations in TP, and espe- cially CHL, from an equation that defines them purely in terms of the phosphorus supplied to lakes annually from external sources.
Whatever refinements and modifica- tions are needed in the future, we believe that the set of relations presented in Fig. 5 provides both insight into the workings of lake ecosystems and information of practical value. The concept that phos- phorus supplied to lakes plays a unique role in controlling phytoplankton produc- tion is strongly supported and on a firm quantitative basis. Of both theoretical and practical interest is the indication that our choice of response parameters may also have defined a useful set of readily determinable descriptive keys to the trophic state of lakes. Winter total phosphorus seems to offer the maximum return of information for effort invested. The high correlations for the TPurAllnp and CHLsTP w equations strongly suggest
144 Oglesby and Schaffner
that TP, is a valid index of the annual rate of phosphorus supplied and of that available to the phytoplankton com- munity for use during the growing sea- son. The very few measurements of win- ter total phosphorus required to develop these regressions showed this to be a rela- tively conservative property of lakes, one that requires comparatively little sam- pling and analysis to define.
In discussing the relation of the mor- phoedaphic index to fish production in lakes Ryder et al. (1974) commented on the increase in generality and utility af- forded by relations that are independent of the typological paradigm. All of the functions developed for New York lakes have this virtue, being independent of both species composition and the classi- cal trophic state terminology.
At a time when legislative mandates, appropriations of governmental funds, and the vociferousness of environmen- talists all demonstrate a high level of con- cern about the eutrophication of lakes in North America, it is especially important that management decisions be made and actions taken on as rational and quantita- tive a basis as possible. The use of sim- ple, readily determinable (with the ex- ception of A’,,,& components in our model, and the fact that they are couched in terms of factors (chlorophyll and transparency) directly perceived by the public, suggests that this model may be useful in develop- ing strategies for water quality manage- ment.
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Submitted: 8 September 1976 Accepted: 8 April 1977
