A comparison of internal phosphorus loads in lakes with anoxic hypolimnia: Laboratory incubation versus in situ hypolimnetic phosphorus accumulation1

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1 1160 Notes Limnol. Oceanogr., 32(S), 1987, 1160-l , by the American Society of Limnology and Oceanography, Inc. A comparison of internal phosphorus loads in lakes with anoxic hypolimnia: Laboratory incubation versus in situ hypolimnetic phosphorus accumulation1 Abstract-Phosphorus release rates from anoxic sediments were determined from laboratory incubations of minimally disturbed sediment cores from seven lakes. These release rates were multiplied by an anoxic factor, based on the area and duration of hypolimnetic anoxia, to estimate internal phosphorus load. Despite individual variation, the experimentally derived internal loads were not significantly different from internal loads determined from hypolimnetic phosphorus increases at the end of summer stratification, except at internal loads ~50 mg m-2 yr-i. Therefore, laboratory release rates can help predict internal loads after lake conditions have changed, e.g. after in-lake restoration measures. Lakes with anoxic hypolimnia during stratification are known to accumulate total phosphorus in the hypolimnion. Simultaneous increases of reduced substances (ferrous iron, manganese, and hydrogen sulfide) and laboratory release experiments (Mortimer 1941, 1942; Niirnberg et al. 1986) suggest that much of this phosphorus is derived from the anoxic sediment surface. Since the amount released can be as high as external phosphorus inputs, this internal input must be considered in predictive models of lake phosphorus concentrations (Niimberg ). Internal phosphorus load (mg m-2 lake surface area per year or per summer) can be determined in at least three different ways, depending on which variables are available: from phosphorus profiles in the anoxic hypolimnion at the end of stratification and hypolimnetic morphometry (this approach is often considered to produce the best estimates); from laboratory incubation experiments with sediment cores, combined with calculations taking into account the duration and spatial extent of anoxia in the lake; and from an annual phosphorus budget, as the difference 1 Supported by a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada to G.K.N. of measured phosphorus retention, (R = input - output)/input, and phosphorus retention predicted for lakes without anoxia, R = 15/(18 + q,), multiplied by external load (Niimberg ). The last approach was evaluated previously (Niimberg ), and the results obtained compared well with the first approach, although some deviations occur (Stauffer pers. comm.). The present investigation was conducted to evaluate whether laboratory release rate experiments and information on anoxia can be used to reliably calculate internal phosphorus load. Laboratory and in situ release rates have been measured before. The overall comparison yielded ambiguous results, although there was a tendency for experimental release rates to be lower than in situ release rates (see Numberg et al. 1986). I thank P. Dillon, B. LaZerte, and R. Stauffer for input to this manuscript. In this study, in situ loads were determined as the difference between maximum hypolimnetic P mass and background P mass just before the onset of anoxia. (This background differs from that of Niirnberg et al , where average midsummer epilimnetic phosphorus concentrations were used instead.) In lakes where the anoxicoxic boundary is much deeper than the thermocline, maximum internal load was calculated from profiles at the very end of summer stratification or from the increase in epilimnetic P concentration at fall turnover (all but Lake Waramaug, Wononscopomuc, and St. George). In lakes where the anoxicoxic boundary approaches the thermocline very closely, apparent hypolimnetic P mass is largest in the summer before onset of thermocline erosion; if internal load is determined at the very end of summer stratification instead, it can be greatly underestimated (Table 1). Underestimation is probably due to the mixing of high concen-

2 Notes 1161 Table 1. Decreases of in situ internal loads, if de- Table 2. Average experimental release rates (RR, termined at the very end of summer stratification (late mg m-2 d-l) of phosphorus from the deepest spot in L,,,) rather than at maximum hypolimnetic P mass seven lakes, standard error of the mean (SE), and samduring summer (see Table 4), and corresponding an- ple size (n). (Lakes Waramaug and Red Chalk: three oxic factor. sample sites.) t 1985-f -f t Late L,,, (mg mm2 summer-l) Decrease St. George, east Waramaug* Wononscopomuc, large Wononscopomuc, small Anoxic factor (d) * No fall data available for and. t Hypolimnetic water was withdrawn during anoxic summer stratification to decrease anoxia and internal phosphorus load. trations of hypolimnetic phosphorus into the eroding epilimnion with subsequent loss through the outflow. In these lakes, a more accurate estimate of in situ load is probably only possible by monitoring in-lake concentrations as well as concentrations and volume of the outflow during the entire period of thermocline erosion (Niirnberg 1985). Experimental release rates (mg m-2 d-l) were determined in undisturbed, anoxic cores (redox potential of the sediment-water interface ~200 mv) from 1 to 3 sites in each of seven lakes. Incubations were done at constant temperature (1 l C) and with large sample numbers (3-l 6) to provide a precise estimate of in-lake release rates (see Niirnberg et al. 1986). The average rates differed significantly from lake to lake and ranged from 0.1 to 8.1 mg m-2 d-l (Table 2). Rates in cores derived from different depths of Lake Waramaug and Red Chalk, east (Table 3), were averaged, but those of Lake Wononscopomuc were treated separately because they came from two distinctly different basins. The data for Chub Lake indicate Lake RR SE n Chub Gravenhurst Bay Pt Red Chalk, east St. George, east Waramaug Wononscopomuc, large Wononscopomuc, small that the thermal structure of the lake and redox state can affect release rate, and the rate during the anoxic stratification period (2 1 September) was selected. Several workers have determined that release rate is temperature-dependent (Yoshida ; Psenner ; Riley and Prepas ), and there is some evidence of varying rate with duration of anoxia (Kamp-Nielsen 1974). If possible, cores were sampled on several occasions and at several depths (Chub, Red Chalk, Waramaug, Wononscopomuc) or at about the deepest location during anoxic stratification (Lake St. George, Gravenhurst Bay, Pt- 10). Release rates of P were converted into Table 3. Average experimental release rates (RR, mg m-2 d-l) and sources of variability: different depths, location, time, and oxygen concentration (anoxic, except Chub Lake 14 March: 4 mg 0, liter- ). Depth RR SE n Sampling date Lake Waramaug 7.5* Jul f Jul t Aug 85 Red Chalk, east Ju Ju Jul Aug84 Chub Lake Sep * Nov Mar85 * West basm. t East basin. $ Thermoclme eroded to 1 m above sediment.

3 1162 Table 4. Anoxic factor (d) and internal loads of phosphorus (mg m-2 summer ) for several years in the seven study lakes. In situ-internal load derived from maximal increases in hypolimnetic phosphorus mass or epilimnetic increases at fall turnover (Gravenhurst Bay: fall 198 1, ; Chub: average of both estimates); cores-derived from incubation experiments (Table 2) ) Internal load Anoxic factor In situ Cores Chub* Gravenhurst Bayt Pt-10* Red Chalk, east* St. George, east$ Waramaug Wononscopomuc, large# Wononscopomuc, small# * P. Dillon unpubl. data. t K. Nicholls and G. Robinson unpubl. data. * D. McQueen unpubl. data. 0 Lake Waramaug Task Force, Inc. unpubl. data internal phosphorus loads through multiplication by an anoxic factor : Internal load = release rate X anoxic factor. (1) The anoxic factor was computed from the anoxic area, duration of anoxia, and lake surface area: Anoxic factor = (duration x anoxic sediment area) + lake area. (2) The anoxic factor represents the number of days that a sediment area, equal to the whole-lake surface area, is overlain by anoxic water (< 1 mg O2 liter- at 1 m above the sediment) and can be used for lake-tolake comparisons. The factor can be determined from lake morphometry and oxygen profiles at the deepest location during the stratified period. For comparison of internal loads in this study, anoxic factors were calculated from the period ending with the date used to determine in situ load (Table 4). It is advisable to measure oxygen concentrations from deep and shallow sites, since anoxia appears to start and end earlier at shallower sites (Fig. 1 and Stauffer 1985). Therefore, in the large Lake Waramaug and in Lake Wononscopomuc s distinct basins, oxygen profiles from several locations were used in the calculations. Internal loads derived from laboratory release experiments and the anoxic factor were compared to internal loads computed from field data on maximum increases of hypolimnetic phosphorus mass. The data for 34 lake-years of seven lakes are listed in Table 4. In general, the difference between loads is not significant (via either the nonparametric Wilcoxon paired-signed ranks test or the t-test on log-transformed variables). At internal loads ~46 mg m-2 summer-, however, laboratory-based loads are significantly lower than in situ loads (Wilcoxon P < 0.02); the difference is not significant at higher loads. The regression between in c 11 Hypolimnetic water was withdrawn durmg anoxic summer stratification (Niimberg et al. 1987). # E. Davis unpubl. data.

4 Notes 1163 Lake Waramaug S+a+lori5 Depth (m) Fig. 1. The dependence of the oxycline (anoxic-oxic boundary) on water depth and time: shallow sites become anoxic earlier at the same depth (Lake Waramaug, monthly and annual averages for -, all stations are in connected basins), but also destratify and re-aerate earlier (Lake Magog, 198 1, stations are in the same basin). situ release rates and those measured with cores is highly significant after logarithmic transformation to stabilize the variance (r2 = 0.85, P < , n = 34, Fig. 2). The regression line falls close to the 1: 1 line; however the slope is significantly < 1. At internal loads above 46 mg rnp2 summer-l, the regression line coincides with the 1 : 1 line [log(in situ) = 0.63 l(o.471 SE) (0.193 SE) log(cores), n = 18, r2 = 0.47, P < c er t ain lakes seem to exhibit the same tendency, i.e. either positive or negative differences for all years (Table 4) which might indicate that the deviations are due to errors in estimating laboratory-derived loads. The tendency for in situ loads ~46 mg m-2 summer- to be underestimated by laboratory-based loads may be an artifact: cores from oligotrophic lakes could have become oxic for short periods of time after sampling because of low biological and chemical oxygen demand. Determination of the anoxic factors could also be erroneous, since they were determined at only one site per lake, except in Lake Waramaug. Possible anoxia of extended shallow areas could lead to higher anoxic factors and comparably higher

5 1164 Notes Experimental (mg m-2summer- ) Fig. 2. Comparison of internal load (mg mm2yr- ) derived from increases in hypolimnetic phosphorus mass (in situ) and experimentally derived internal load (cores) after logarithmic transformation. 1: 1 line is shown, but best fit (r2 = 0.85, y1= 34) is to the equation log(in situ) = 0.5 l(o.11 SE) (0.06 SE) log(cores). laboratory-derived loads. In Lake Waramaug ( 198 5) the laboratory-based internal load would be underestimated by 10% if only the profiles of the deepest station were used. Errors may also arise in determining internal load from hypolimnetic phosphorus mass. In these calculations, external loads are assumed to be constant over the period of anoxia (i.e. summer and fall), but variations may be significant in lakes experiencing anthropogenic inputs during summer. Enhanced seasonal input can be expected, especially in the remote oligotrophic lakes. Furthermore, the hypolimnetic phosphorus concentration above a certain background concentration (i.e. hypolimnetic phosphorus concentration before the onset of anoxia, usually shortly after spring turnover) is considered to be entirely controlled by sediment release. Phosphorus release from settling plankton, decaying macrophytes, and oxic sediments might result in slight overestimates of internal load from anoxic sediment surfaces. Despite all the possible error sources, the comparison reveals a good correlation of internal phosphorus loads determined from increases in hypolimnetic phosphorus mass on the one hand and laboratory release experiments combined with anoxic factors on the other. Laboratory release rates, therefore, can be used to describe processes in the past and present if reliable oxygen profiles in the anoxic period are available and adequate experiments are conducted, even if phosphorus profiles are not available. This approach is particularly valuable when predictions of future internal phosphorus loads are required and can help in evaluating the potential effects of several in-lake restoration techniques such as sediment dredging, hypolimnetic withdrawal, and aeration. Faculty of Science York University Downsview, Ontario M3J lp3 References Gertrud K. Niirnberg2 KAMP-NIELSEN, L Mud-water exchange of phosphate and other ions in undisturbed sediment cores and factors affecting the exchange rates. Arch. Hydrobiol. 73: MORTIMER, C. H , The exchange of dissolved substances between mud and water in lakes. 1, 2, 3, 4. J. Ecol. 29: ; 30: NÜRNBERG, G. K.. The prediction of internal phosphorus load in lakes with anoxic hypolimnia. Limnol. Oceanogr. 29: Availability of upwelling phosphorus from iron-rich anoxic hypolimnia. Arch. Hydrobiol. 104: , R. HARTLEY, AND E. DAVIS Hypolimnetic withdrawal in two North American lakes with anoxic phosphorus release from the sediment. Water Res. 21: in press. -, M. SHAW, P.J. DILLON,AND D.J. MCQUEEN Internal phosphorus load in an oligotrophic Precambrian Shield lake with an anoxic hypolimnion. Can. J. Fish. Aquat. Sci. 43: PSENNER, R.. Phosphorus release patterns from sediments of a meromictic mesotrophic lake (Piburger See, Austria). Int. Ver. Theor. Angew. Limnol. Verh. 22: l-10. RILEY, E. T., AND E. E. PREPAS.. Role of internal phosphorus loading in two shallow, productive lakes in Alberta, Canada. Can. J. Fish. Aquat. Sci. 41: STAUFFER, R. E Lateral solute concentration gradients in stratified eutrophic lakes. Water Resour. Res. 21: YOSHIDA, T.. On the summer peak of nutrient concentrations in lake water. Hydrobiologia 92: Submitted: 3 July 1986 Accepted: 17 March Mailing address: % Ontario Ministry of the Environment, P.O. Box 39, Dorset, Ontario POA 1EO.