Our success was due in no small part to

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1 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 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 development 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 represent 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 responses of lakes to mixed zone loading (A,,) as an interrelated series of simple 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 information on them could be obtained from the literature or by actual measurement, and of their practical value. The concentration of phytoplankton pigments in the upper part of the water column during summer, winter phosphorus concentration 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 Resources 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 concentrations 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 parameters 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. Because interrelations between the various response parameters were to be examined, we also collected information to describe several other lakes for which loading estimates could not be calculated. Our success was due in no small part to 1,IMNOLOGY AND OCEANOGRAPHY 135 JANUARY 1978, V. 23(l)

2 136 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. Chamberlain. P. J. Dillon s helpful comments on the manuscript are also acknowledged. Limnology of study lakes Sixteen lakes in central New York State, representing wide ranges of probable nutrient input, size, depth, and hydrologic 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 parameters 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 between the lakes by more than a factor of 10, surface areas by as much as two, and volumes by over three orders of magnitude. The longest mean hydraulic retention time (Seneca) is 23 times greater than the shortest, for Honeoye. Population and land use in the basins were described 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 intensively 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 during at least one determination of winter total phosphorus (samples collected April) and two of summer chlorophyll and transparency (17-18 July and August), along with other limnological measurements. Additional data for (G reeson 1971) and for (Mills unpublished) were available for Oneida Lake. All samples were taken with PVC Van Dorn bottles and transferred to phosphorus-free polyethylene containers. 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 measured 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 filtration onto a Reeve Angel 984 AH glassfiber filter, according to the method of Strickland and Parsons (1972) and concentrations calculated by the equations of Lorenzcn (1967). Measurements were made in a I-cm cell with a spectrophotometer (Turner 330). Methods for phosphorus analysis were given by Schaffner and Oglesby (1978); absorbance was measured in a lo-cm cell in a spectrophotometer (Beckman DU). Those lakes that have been comprehensively 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

3 Phosphorus loadings to lakes lakes. There are large differences in NOB--N and reactive silicon between lakes. The former decreases markedly in those Finger Lakes west of Keuka. Summer nitrate levels in Oneida (Greeson 1971), Canadarago, and Waneta also appear to be low. Limited data indicate that of the Finger Lakes the two largest, Seneca and Cayuga, have the lowest epilimnetic reactive silicon concentrations. We find highly persuasive the published evidence and arguments that phosphorus generally controls phytoplankton production and standing crops in temperate lakes. Nevertheless, nitrogen 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 Oglesby (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 depleted 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 fixation (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 examplc, 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 replaced by Melosira, but during this change NO,--N showed little or no increase. Increases in phytoplankton stand- Table 1. Seasonal average concentrations (mg m-3) for 1972-, integrated over depth and time, of soluble reactive phosphorus (SRP), nitrate nitrogen (NO,--N) and soluble reactive silicon (SRS) in the upper 10 m of the water column (data from Mills 1975) ~- --_---- _ Lakes* L -ow _---. _-.--_--- CO HE ~--- SK -- Summer SRP NO--N SR 3 56% 737 1, Fall SRP NOi-N SRS Winter SRP NOi-N ,657 1,029 SRS , Spring SRP N03-N SRS Summer SRP NO;-N SRS ---~_ _ _--. *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 summer. Patterns of seasonal change with very low (ca. < l-2 mg mv3) soluble reactive phosphorus levels occurring at intervals during summer are themselves strong presumptive evidence of limitation 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 alkaline 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 phosphorus concentrations increased have also been found for summer communities in Conesus, Hemlock, Owasco, and Skaneateles lakes. Among the limnological factors indicative of lake trophic state, other than nu-

4 138 Oglesby and Schaffner 25 t x r2 = 0.78 / 01 1 I I L (mg.m 3.yr-1) f-imp Fin. 1. Regression of mean summer Chl a + pheopigment for upper 10 m of water column (CHL,) on annual mixed zone loading of biologically available phosphorus (A,,). trient supply and phytoplankton standing crops which are discussed below, are dissolved 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 oxygen minima above 6 mg liter-l. Concentrations 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 remaining lakes, Waneta and Lamoka had hypolimnetic minima of l-3 mg liter- in August and Oneida exhibits transient anaerobic conditions in its deeper waters during periods of wind calm. Dissolved oxygen in Canadarago Lake was not measured. Comparing historical information 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 differences in productivity between lakes or of changes in productivity for any single lake over time. About 400 taxa of phytoplankton were identified during the 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 ( : Howard 1963; 196&1970: Peterson 1971.) However, even this limited information shows the differences possible between years having presumably 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 supply of biologically available phosphorus (BAP) to lakes, together with values determined for 13 New York lakes. The biological response of receiving systems to this input was measured in terms of phytoplankton standing crop, as estimated 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 rationale 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 recreation 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 published data are expressed as concentrations at O-IO-m depths, and this gave a relation that could be tested for other lakes. Ten meters is also a reasonable approximation of the depth of the thermo-

5 Phosphorus loadings to lakes Table 2. Summer pigment (CHL,) and winter phosphorus (TP,) concentrations for New York lakes. CHLs* No. of SE of No. of SE of -3-3 Lake Year mg m samples mean mg m samples mean Conesus Conesus Conesus Hemlock Hemlock Hemlock Canadice Honeoye Canandaigua Keuka Keuka Seneca Seneca Seneca Cayuga Cayuga Cayuga Wwa Owasco Owasco Owasco Skaneateles Skaneateles Skaneateles Otisco Oneida Oneida Oneida Oneida Oneida Canadarago Otsego Waneta Lamoka : E$ ;;;;I t ::2" TPw? P ;:: ** : i.i ; ; " *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.?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). i 2 i ; 1 Cline, and a significant part of the euphotic zone for moderate-sized lakes in temperate latitudes. The pigment data are piesented in Table 2. Standard errors of the means were calculated for values represented by more than four samplings. The relation between phosphorus loading to the mixed zone (A,,) and standing crop (CHL,) is shown in Fig. 1. There is no reason to think that A,,, as it affected phosphorus availability during the summer growing season, varied appreciably during We therefore assume the scatter about the regression line is a function of changes in standing crop in response to factors other than phosphorus loading, such as available light, degree of mixing, differences

6 140 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 productivity during 1957 and 1958 in Cayuga Lake did not include CHL, measurements. However, if the chlorophyll averages for Cayuga arc used as reference points, and if productivity is assumed proportional to CHLs, a variation of the magnitude Howard described would have produced mean summer chlorophyll concentrations ranging from about mg rnm3. In other words, one piece of evidence indicates that factors other than phosphorus loading could have produced variations in the average 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 winter (TP,) has been used as an index of the phosphorus that will be available to support phytoplankton growth the following summer. Sawyer (1947) used this in a rather general (and often misquoted) way TP, (mg KS) Fig. 3. Regression of CHL, on TP, for deeper New York lakes extrapolated to include range of values for Lake Washington ( ). 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 Dillon and Rigler (1974a) with data for North America and Europe. Both established relationships that were linear-but only on a log-log plot-thus imposing very large confidence intervals, especially at higher TP, and chlorophyll concentrations, 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 ; 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 characteristic of lakes with very low TP, concentrations. 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 phytoplankton production. The unique historical data available for J 10

7 Phosphorus loadings to lakes Lake Washington (Edmondson 1972) allowed 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 substantially ( ) and then decreased as sewage effluents were diverted ( ). 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 different 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 following growing season. Phosphorus loading and winter total phosphorus Given the previously established relations 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 annually to the lake from outside sources. As with the CHL&,, regression, the high degree of correlation indicates that effects of depth, and indirectly of hydraulic retention time, are adequately accounted for by the way in which phosphorus supply to the New York lakes is calculated. The intercept of this regres- 0 1 I I 1 1 I np (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 replenishment cycle is appropriate for describing 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 chlorophyll concentration. With values influenced 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 transparency as phytoplankton standing crop is changed (Edmondson 1972). The following relation showing the dependence of SDT, on CHL, was derived from about 400 observations on New York lakes, and an almost equal number from Lake Washington (Edmondson 1972) added to define the high

8 142 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 = 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 dependence of transparency on chlorophyll concentration is what might be expected according to the Beer-Lambert law if the primary effect of phytoplankton on transparency were absorption of light. A similar relation of SDT, to TP, is to be expected in view of the high linear correlation 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 = loglo X; r2 = 0.81 where Discussion Y is SDT, and X is TP,. The regression equations developed in this study can be connected in a logical sequence (Fig. 5) if the following assumptions are made: that the concentration of phytoplankton pigments in summer is a function of phosphorus, expressed as a biologically available composite 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 growing season; and that transparency, as measured by the Secchi disk, is a function of phytoplankton abundance and this is also reflected in an indirect relation to TP,. The high correlation coefficients indicate that these are valid working postulates 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 calculating loading provides not only a rational but also a simple way of dealing with the depth factor. With mean hydrau- lic retention times (T,) for the lakes ranging from yr, the high correlation for our CHLsR,, regression indicates that water renewal time is probably accounted 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 extrapolating 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 yr. Our general approach to the response

9 of lakes to nutrient loading in some ways resembles that used by Shannon and Brezonik (1972) in their study of 55 shallow 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, specific 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 expressed as a log function) applicable to New York lakes. However, we found no significant correlation between conductivity 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 variable has the advantage of simplicity of presentation, this is offset, especially where a group of new relations is proposed, by the complexity of calculation and interpretation. We believe that by leaving the response parameters and interrelations 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 decisions. Whether the relations established here for a group of New York lakes can bc cxtrapolated to other situations remains untested. 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 mg rna3 yr-l. Deviations from the CIIL,A,, and CHL,TP, regressions may occur where turbidity due to allo- Phosphorus loadings to lakes chthonous materials or resuspended bottom sediments, or both, persists during the growing season or where humic substances 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 eutrophic lakes in Great Britain and Europe), adjustments will have to be made in the period for which CHL, and SDT, are calculated. In situations where nutrient inputs undergo sudden, large changes there may be a period of disequilibrium between phosphorus and standing crop that will cause deviations from our regressions. However, the response of Lake Washington to changes in phosphorus input suggests that equilibria may be established rapidly even in lakes with relatively long water retention times. We also expect variations from our regressions 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 especially 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 phosphorus supplied to lakes plays a unique role in controlling phytoplankton production 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

10 144 Oglesby and Schaffner that TP, is a valid index of the annual rate of phosphorus supplied and of that available to the phytoplankton community for use during the growing season. The very few measurements of winter total phosphorus required to develop these regressions showed this to be a relatively conservative property of lakes, one that requires comparatively little sampling and analysis to define. In discussing the relation of the morphoedaphic index to fish production in lakes Ryder et al. (1974) commented on the increase in generality and utility afforded 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 classical trophic state terminology. At a time when legislative mandates, appropriations of governmental funds, and the vociferousness of environmentalists all demonstrate a high level of concern 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 simple, readily determinable (with the exception 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 developing strategies for water quality management. References BACHMANN, R. W., AND J. R. JONES Phosphorus inputs and algal blooms in lakes. Iowa State J. Res. 49(2): DIRGE, E. A., AND C.JUDAY A limnological study of the Finger Lakes of New York. Bull. U.S. Bur. Fish. 32: ,AND Further limnological observations on the Finger Lakes of New York. Bull. U.S. Bur. Fish. 37: CARLSON, R. E A trophic state index for lakes. Limnol. Oceanogr. 22: CHAMBERLAIN, H. D A comparative study of the zooplankton communities of Skaneateles, Owasco, Hemlock, and Conesus Lakes. Ph.D. thesis, Cornell Univ. 266 p. DILLON, P. J The phosphorus budget of Cameron Lake, Ontario: The importance of flushing rate to the degree of eutrophy of lakes. Limnol. Oceanogr. 20: AND F. H. RIGLER. 1974a. The phosphorus-chlorophyll relationship in lakes. Limnol. Oceanogr. 19: ,AND h. A test of a simple nutrient budget model predicting the phosphorus concentration in lake water. J. Fish. Res. Bd. Can. 31: EDMONDSON, W. T Nutrients and phytoplankton in Lake Washington. Am. Sot. Limnol. Oceanogr. Spec. Symp. 1: GODFREY, P. J Spatial and temporal variation of the phytoplankton in Cayuga Lake. Ph.D. thesis, Cornell Univ. 512 p. GREESON, P. E Lirnnology of Oneida Lake with emphasis on factors contributing to algal blooms. U.S. Geol. Surv. Open-File Rep. 185 p. GRIFFIN, K. C Alkaline phosphatase as an ecological parameter in Cayuga Lake. M.S. thesis, Cornell Univ. 83 p. HAMILTON, D. H., JR., AND J. P. BARLOW Eutrophication of water resources of New York State-a study of phytoplankton and nutrients in Lakes Cayuga and Seneca. Cornell Univ. Water Resour. Center Publ p. HETLING, L. J., AND R. M. SYKES Sources of nutrients in Canadarago Lake. N.Y. State Dep. Environ. Conscrv. Tech. Pap p. HOWARD, H. H Primary production, phytoplankton and temperature studies of Cayuga Lake, New York. Ph.D. thesis, Cornell Univ. 180 p. LOHENZEN, C. J Determination of chlorophyll and phco-pigments: Spcctrophotometric equations. Lirnnol. Oceanogr. 12: MILLS, E. L Phytoplankton composition and comparative limnology of four Finger Lakes, with emphasis on lake typology. Ph.D. thesis, Cornell Univ. 316 p. -, AND R. T. OGLESBY Five trace elements and vitamin Br2 in Cayuga Lake, New York. Proc. 14th Conf. Great Lakes Res : OGLESBY, R.T., AND W.R. SCHAFFNER The response of lakes to phosphorus, p Zn K. S. Porter [ed.] Nitrogen and phosphorus-food production, waste and the environment. Ann Arbor Sci AND E. L. MILLS Nitrogen, phosphorus and eutrophication in the Finger Lakes. Cornell Univ. Water Resour. Mar. Sci. Center Tech. Rep p. PETERSON, B. J The role of zooplankton in the phosphorus cycle of Cayuga Lake. Ph.D. thesis, Cornell Univ. 131 p. - J. P. BARLOW, AND A. E. SAVAGE The physiological state with respect to phosphorus of Cayuga Lake phytoplankton. Limnol. Oceanogr. 19: RAWSON, D. S Mean depth and the fish production of large lakes. Ecology 33:

11 Phosphorus loadings to lakes * Morphometry as a dominant factor in the productivity of large lakes. Int. Ver. Theor. Angcw. Limnol. Verh. 12: RYDER, R. A., S. R. KEnn,K. ILLOFTUS, AND H. A. REGIER The morphoedaphic index, a fish yield evaluator-review and evaluation. J. Fish. Res. Bd. Can. 3 1: SAKAMOTO, M Primary production by phytoplankton community in some Japanese lakes and its dependence on lake depth. Arch. Hydrobiol. 62 : l-28. SAWYER, C. N Fertilization of lakes by agricultural and urban drainage. J. N. Engl. Water Works Assoc. 6 1: SCIIAFFNER, W. R., AND R. T. OGLESBY Phosphorus loadings to lakes and some of their responses. Part 1. Limnol. Oceanogr. 23: 12O , AND-. In press. Limnology of eight Finger Lakes. N.Y. Lakes Monogr. Ser. Academic. SHANNON, E.E., AND P.L. BREZONIK Relationships between lake trophic state and nitrogen and phosphorus loading rates. Environ. Sci. Technol. 6: STEWART, K. M., AND S. 1. MARKELLO Seasonal variations in concentrations of nitrate and total phosphorus and calculated nutrient loading for six lakes in wcstcrn New York. Hydrobiologia 44: STKICKLAND, J. D., AND T. R. PARSONS A practical handbook of seawater analysis, 2nd ed. Bull. Fish. Res. Bd. Can THIENEMANN, A Der Bau des Seebeckens in seiner Bedeutung fur des Lebel im See. Verh. Zool.-Bot. Ges. Wien 77: U.S. ENVIRONMENTAL PROTECTION AGENCY. 1974a. Report on Cayuga Lake, Cayuga, Seneca, and Tompkins Counties, New York. EPA Region 2. Working Pap Natl. Environ. Res. Center. 29 p b. Report on Seneca Lake, Schuyler County, New York. EPA Region 2. Working Pap Natl. Environ, Res. Center. 57 p. VOLLENWEIDER, R. A Water management research. OECD Paris. DAS/CSI/ p. AND P. J. DILLON The application of thb phosphorus loading concept to eutrophication research. NRCC Natl. Res. Count. Can. 42 p. Submitted: 8 September 1976 Accepted: 8 April 1977