Spatial and temporal variation in hypolimnetic oxygen deficits of a multidepression lake

Transcription

1 740 Notes Limnol. Oceanogr., 32(3), 1987, , by the American Society of Limnology and Oceanography, Inc. Spatial and temporal variation in hypolimnetic oxygen deficits of a multidepression lake Abstract-Marked differences in areal hypolimnetic oxygen deficits were found among three depressions of a single lake. These differences were similar to those measured 11 yr before and indicate that regional patterns of nutrient input, production, and eutrophication exist. The patterns are related to lakeshore land use and development. The rate of eutrophication has accelerated across the lake over the 11 -yr span. This increase in eutrophication rate would not have been detected had not the hypolimnetic oxygen deficit data also taken into account the differences in hypolimnetic temperatures between the two studies. In fact, without considering temperature, opposite conclusions would have been reached; i.e. that the rate of eutrophication was lessening. The rate of oxygen depletion in the hypolimnion of a stratified lake has been assumed to be proportional to the rate of primary production of organic matter in the epilimnion (Hutchinson 1938; Strom 193 1). This organic matter ultimately sinks into the hypolimnion where decomposition depletes the oxygen acquired during spring overturn. The quantity of epilimnetic primary production is a function of nutrients supplied through internal loading at overturn, supplemented by external nutrient loading from the watershed. Welch and Perkins ( 1979) utilizing a sample of 26 lakes, have shown the hypolimnetic oxygen deficit to be correlated positively with phosphorus loading. The OECD Cooperative Program on Eutrophication (OECD 1982) reported a strong relationship between phosphorus loading and oxygen depletion rates of North American and Nordic lakes (r = 0.60, P < 0.05). Other workers have cited phosphorus loading as the most frequent correlate with eutrophication (Chapra and Reckhow 1979; Vollenweider 1976; Dillon and Rigler 1974). Consequently, one should be able to use a changed rate of hypolimnetic oxygen depletion to characterize cultural eutrophication. Hutchinson (1957) proposed a generally accepted trophic classification based on oxygen depletion rates. Various changes in watershed use increase nutrient input, epilimnetic production, and, consequently, hypolimnetic oxygen depletion. The impact of watershed nutrient input can be assessed by a series of direct measurements of phytoplankton primary production. Such measurements have high short-term variability and are moderately difficult and expensive. In contrast, the areal hypolimnetic relative oxygen deficit (AHOD) has low short-term variability, is easy to measure, and provides a timeintegrated estimate of the epilimnetic primary production. In 197 1, I applied the AHOD method to three depressions in Douglas Lake, Michigan, to determine if one could detect spatial patterns in eutrophication caused by lakeshore development and use (Lind 1978). My data supported the hypothesis of a greater AHOD rate in Fairy Island depression, nearest the region of human lakeshore use (Fig. 1). Subsequently, Cornett and Rigler (1979, 1980) Charlton (1980) and Fulthorpe and Paloheimo (1985) identified and evaluated three factors-in addition to those described by Hutchinson ( 195 7) -that sig- nificantly affect the rate of hypolimnetic oxygen depletion. These three factors are: the flushing of productivity from the lake without sedimentation into the hypolimnion, the mean thickness of the hypolim- nion, and the mean temperature of the hypolimnion. Failure to consider flushing is most significant in lakes with short water retention times. Flushing probably can be ignored in lakes where the water retention time is of the order of years. Hutchinson ( 1938) recognized the need to consider the relative volumes of the epilimnion and the hypolimnion. Thus, he modified the volumetric method of Strom (193 1) to an areal basis-the method that prevailed for 40 yr. The mean hypolimnetic thickness was thought to be accounted for by calculating areal deficits. Recent investigations show this assumption to be false because of dif-

2 Notes 741 Fig. 1. Douglas Lake, Michigan. Types of lakeshore land use and the three depressions considered in this study are shown. FID-Fairy Island depression; GPD-Grapevine Point depression; SFBD-South Fishtail Bay depression. ferent organic matter decomposition rates in the water column vs. the sediments. Because of sediment anaerobiosis, the oxygen demand for the decomposition of organic matter reaching the sediments is nil. Cornett and Rigler (1980) showed that when other factors are equal, deeper hypolimnia have greater oxygen deficits. Charlton (1980) used data from the Laurentian Great Lakes to show a hyperbolic relationship between relative (i.e. standardized for temperature and organic input) AHODs and mean hypolimnion thickness. He found that this model held also for small lakes. The critical hypolimnion thickness was 50 m. In hypolimnia deeper than 50 m, most of the epilimnetic production fully decomposes in the water column, thereby contributing to the oxygen deficit. In progressively thinner hypolimnia, more organic matter settles to the mud and does not contribute to pelagic oxygen depletion. Consequently, the AHOD underestimates epilimnetic production in these lakes. Finally, and obviously, the hypolimnetic temperature significantly affects the metabolic rate of the decomposers. This consideration becomes increasingly important as limnological investigations include more lakes that deviate from the north tem- perate, second-order lake typology where the hypolimnetic temperature is near 4 C. Charlton (1980) corrected for this effect by assuming a temperature coefficient (Qlo) of two. I undertook a second study of Douglas Lake in 1982 for the dual purpose of confirming the conclusions regarding spatial patterns of eutrophication and of determining if the rate of eutrophication for the lake had changed during the 11 -yr period since the first study. This period was a time both of continued lakeshore development and of increased environmental awareness, that included implementation of water quality codes at all governmental levels. This note has two purposes. It reports the results of the second study and compares them with the first study for the purpose of determining spatial and temporal variation in eutrophication. Secondly, and of more general interest, it describes how the additional factors of temperature and hypolimnion mean thickness affect the use of the AHOD for comparative purposes. This data set demonstrates especially the importance of including temperature in reaching conclusions regarding differences in AHODs and eutrophication rates, and furthermore,

3 742 Notes Table 1. igan. Physical features of Douglas Lake, Mich- Location N, 84 5O W Lake area 15 km2 Watershed area 56 km2 Maximum depth (Z,,,) 27 m Mean depth (Z) 5.5 m Volume 8.3 x 10 m3 Water retention time 3 v Lake-associated dwellings 244 Data from a report by Gold et al. for the Umverslty of Mlchgan BIological Statlon. it confirms my original conclusions regarding spatial patterns as influenced by lakeshore land use. Douglas Lake is a medium-sized, secondorder temperate glacial lake located in the northern tip of the lower Michigan peninsula (Table 1). It is the site of the University of Michigan Biological Station (UMBS), which was established in 1909 on South Fishtail Bay (SFBD of Fig. 1). Consequently, this lake (specifically South Fishtail Bay) provides one of the few instances for which long-term limnological data are available. For many years South Fishtail Bay has been the object of investigations of temperature and dissolved oxygen both by researchers and as class projects. Bazin and Saunders (197 1) collated many of these data and constructed a linear model showing that the rate of hypolimnetic oxygen depletion had accelerated through the years. The fact that rates for a single depression were compared over the years negates or at least minimizes the problems of flushing, mean hypolimnion depth, and, to a lesser extent, temper- ature. Any increase in AHOD rate may be attributable to eutrophication brought about by increased or changed lake and lakeshore use. Douglas Lake is unusual because it has multiple depressions. The hypolimnion of each depression is isolated from other hy- polimnia by a shoal reaching upward to the metalimnion or epilimnion. The human activity suspected of accelerating eutrophication occurs on the western and northwestern portions of the lakeshore (Fig. 1). The eastern and southern shorelines, owned by UMBS, are covered with a mixed decid- Table 2. Mean hypolimnion volume-weighted temperature and mean thickness of the three depressions in Douglas Lake during the periods of study. DepressIon FID GPD SFBD Thickness ( Temp ( C) uous-conifer forest that is generally free from human impact. The same depressions were used in and 1982: Fairy Island depression (FID), Grapevine Point depression (GPD), and South Fishtail Bay depression (SFBD) (Fig. 1). Seven samplings were made between 4 June and 4 July This period was selected to assure that rates of oxygen depletion were calculated when sufficient dissolved oxygen was present to support normal oxidative processes. The data (24 June-29 July) indicated that by mid-july hypolimnetic oxygen depletion was suffi- cient to retard the depletion rate. Dissolved oxygen and temperature were measured in situ at l-m intervals through the metalimnion and hypolimnion with a YSI model 54A oxygen electrode-thermistor thermometer. Temperature data were plotted with the upper limit of the hypolimnion defined as that point on the depth- temperature plot where a line extrapolated from the approximately vertical series of hypolimnetic temperatures intersected a line extrapolated from the maximum rate of temperature change in the metalimnion. This method, used by Lasenby (1975) for Ontario lakes, and by me in the previous study, provides the same information as the computer-based solution of Bazin and Saunders (197 1). The defined upper limit of the hypolimnion deepened during the study as warming and mixing of the epilimnion drove the metalimnion downward. AHODs, as defined by Hutchinson ( 1957), were calculated from linear regressions of changes in the total hypolimnetic oxygen content with the method described by Lind (1979). Differences in temperatures among depressions and between the two studies and

4 Notes 743 Table 3. Measured areal hypolimnetic oxygen deficits and these values with compensation for mean hypolimnetic temperature alone, and for the product of mean hypolimnetic temperature and mean hypolimnetic thickness for the three depressions in Douglas Lake for 1971 and DepressIon AHOD* T T(z) AHOD* T T(z) FID GPD SFBD * AHOD as mg 0, cm-l d-l. differences in hypolimnion thickness required compensation. The calculated AHOD was made relative to a mean volume-weighted temperature of 4 C and a hypolimnion mean thickness of 50 m. Fifty meters is taken as the critical depth beyond which all organic matter decomposes in the water column (see Charlton 1980). A temperature coefficient (QlO) of two was assumed. Accepting Charlton s conclusion (Charlton 1980, p. 1538) that... the main factors controlling hypolimnion O2 depletion are multiplicative, not additive the following relationship was used: Relative AHOD = AHODl{2[(T-4)/101} *(Z/50 + 2) where T is temperature ( C) and 2 is water depth (m). These relative AHODs were then normalized to actual hypolimnion thickness by (relative AHOD/SO)(Z). The mean hypolimnion thicknesses of SFBD and FID were similar to each other, but greater than GPD. The depth of stratification was almost the same for all depressions in and Mean volumeweighted temperatures for each depression were much warmer in than in 1982 (Table 2). The areal oxygen content of FID at the beginning of the 1982 study was 5.68 mg cme2. This content was approximately twice that of the other depressions, and this relative difference in oxygen content continued throughout the study. Because of the timing of the sampling, and unlike the study, no decline occurred in AHOD rate due to oxygen limitation near the end of the sampling period. In Table 3, I compare the measured areal rates of change with rates derived by compensating for hypolimnetic mean thickness and mean temperature. Without compensation, FID, near the area of maximum lakeshore land use, had the greatest AHOD in both 1971 and 1982, thus confirming my original hypothesis. Note that the high value for SFBD in was believed to be an aberration (discussed in my 1978 paper) due to localized macrophyte decomposition. And the 1982 rate for SFBD of mg 0, cm-2 was almost exactly that predicted by the model of Bazin and Saunders (197 1). Because their model was based on many years of data for a single depression, variables of flushing and mean hypolimnetic thickness should be nearly constant, and although temperatures may vary considerably from year to year-as shown by my datathe long-term mean temperature for a single depression also should be nearly constant. After compensating for hypolimnion mean thickness and mean temperature, a similar relationship among depressions appears; i.e. regional eutrophication persists. The 1982 AHOD for FID is again greater than for GPD or SFBD, which were similar. Although compensating for thickness and temperature does not change the conclusions regarding spatial patterns, it does reveal an important temporal trend that is the opposite of that without compensation. Without compensation, one would conclude that the eutrophication rate for the impacted FID had declined between and 1982 (0.097 vs mg 0, cm-2) when in fact it had increased markedly from to mg O2 cm-. The reason for the discrepancy, of course, is the great difference in mean temperatures between the two studies (Table 2). The AHOD rate across the lake has increased considerably during this 11-yr interval.

5 744 Notes Comparing the AHOD rates of FID and those of GPD and SFBD indicates that the former continues to receive relatively more organic input, presumably from higher rates of epilimnetic production supported by greater loading of nutrients in the cottagelined western end of the lake. Although natural eutrophication rates would be expected to vary little through such a short timespan, changes in eutrophication rate due to human-induced nutrient loading have been reported (Edmondson and Lehman 198 1). Apparently there has been a significant change in human impact on Douglas Lake since And, this effect is not only localized in the western region of the lake nearer the probable source of human impact; we see a significant change in GPD as well. The anomalous data for SFBD, unfortunately, do not permit comparisons for this bay. However, the fact that the rate for SFBD is almost exactly that predicted (0.060 mg O2 cm-2 measured, mg 0, cmw2 predicted) by the model of Bazin and Saunders (197 1) argues that the problem is lakewide. One would have expected lower rates of oxygen depletion for each depression in 1982 than in 197 1, indicating a real decrease in lake fertility. It has not occurred, despite a concerted effort toward increasing citizen awareness both by the staff of the UMBS and by property owners associations. This effort resulted in modification of land use, regulation of septic tank installation, and citizen awareness of the probable fate of the lake. As demonstrated by Seattle s Lake Washington, eutrophication is reversible, but this reversibility has yet to be achieved for Douglas Lake. Department of Biology Baylor University Waco, Texas Owen T. Lind References BAZIN, M., AND G. W. SAUNDERS The hypolimnetic oxygen deficit as an index of eutrophication in Douglas Lake, Michigan. Mich. Acad. 111: CHAPRA, S. C., AND K. RECKHOW Expressing the phosphorus loading concept in probabilistic terms. J. Fish. Res. Bd. Can. 36: CHARLTON, M. N Hypolimnion oxygen consumption in lakes: Discussion of productivity and morphometry effects. Can. J. Aquat. Sci. 37: 153 l CORNETT, R. J., AND F. H. RIGLER Hypolimnetic oxygen deficits: Their prediction and interpretation. Science 205: , AND The areal hypolimnetic oxygen deficit: An empirical test of the model. Limnol. Oceanogr. 25: DILLON, P. J., AND F. H. RIGLER A test of a simple nutrient budget model predicting the phosphorus concentration in lake water. J. Fish. Res. Bd. Can. 31: EDMONDSON, W. T., AND J. T. LEHMAN The effect of changes in the nutrient income on the condition of Lake Washington. Limnol. Oceanogr. 26: l-29. FULTHORPE, R.R., AND J.E. PALOHEIMO Hypolimnetic oxygen consumption in small lakes. Can. J. Fish. Aquat. Sci. 42: 1493-l 500. HUTCHINSON, G. E On the relation between the oxygen deficit and the productivity and typology of lakes. Int. Rev. Gesamten Hydrobiol. 36: A treatise on limnology, v. 1. Wiley. LASENBY, D. C Development of oxygen deficits in 14 Ontario lakes. Limnol. Oceanogr. 20: LIND, 0. T Interdepression differences in the hypolimnetic areal relative oxygen deficits of Douglas Lake, Michigan. Int. Ver. Theor. Angew. Limnol. Verh. 20: ~ Common methods in limnology. Mosby: OECD Eutrophication of waters: Monitoring, assessment, and control. OECD, Paris. STROM, K. M Fefovatn. A physiographic and biological study of a mountain lake. Arch. Hydrobiol. 22: VOLLENWEIDER, R. A Advances in defining critical loading levels for phosphorus in lake eutrophication. Mem. 1st. Ital. Idrobiol. 33: WELCH, E. B., AND M. A. PERKINS Oxygen deficit-phosphorus loading relation in lakes. J. Water Pollut. Control Fed. 51: Submitted: 8 April 1986 Accepted: 30 January 1987