Phytoplankton Nutrient Status Research Articles

Seasonal patterns for Secchi depth, chlorophyll a, total phosphorus, and nutrient limitation differ between nearshore and offshore in Lake Michigan

Data on Secchi depth, chlorophyll a, total phosphorus (TP), and nutrient status of phytoplankton were collected at five nearshore sites (11–17 m deep) and two offshore sites (>100 m) between the Grand River and Muskegon River outflows during March-December 2014–2018 to describe seasonal patterns and to compare the two depth regions in southeastern Lake Michigan. In contrast to the offshore, where spring chlorophyll a and TP concentrations declined dramatically following the dreissenid mussel expansion, the nearshore region of southeastern Lake Michigan was still characterized by low Secchi depth and elevated chlorophyll a and TP in the spring. During May, median Secchi depth was 5 times higher in the offshore than the nearshore, whereas chlorophyll a and TP were over 9 and 3 times higher in the nearshore, respectively. Even though spring chlorophyll a and TP have declined substantially at some of the nearshore sites compared to 1996, particularly the sites closest to tributary outflows, the overall yield of chlorophyll a per unit TP did not change over time in the nearshore. There were indications of P-deficiency in the nearshore in 2014–2018, but P-deficiency was even more severe in the offshore during the spring where yield of chlorophyll a per unit TP was also lower than in the nearshore. Although dreissenid mussels can be abundant in the nearshore, their populations are patchy and inputs from tributaries provide conditions that apparently dampen any potential filtering impacts of mussels in the nearshore compared to the offshore, especially during the spring.

Effects of maternal phosphorus deficiency on reproductive allocation of the copepod Pseudodiaptomus annandalei Sewell, 1919

The nutrient status of phytoplankton affects the reproductive traits of zooplankton, yet no study has addressed how reproductive allocation varies with phosphorus (P) availability in copepods. We hypothesized that maternal P deficiency reduces reproductive investment in copepods and that interactions exist between maternal and postnatal P condition effects on offspring development. Females of the copepod Pseudodiaptomus annandalei were fed algal prey with different P contents, then allocation of elements and fatty acids in offspring and naupliar development of offspring were measured. Females reduced total reproductive efforts and P investment per offspring under P-deficient conditions. Gross reproduction efficiency of P and the proportion of available maternal P invested in reproduction increased as P consumption decreased, revealing priority of reproductive investment under P deficiency. A high proportion of body docosahexaenoic acid was allocated into reproduction, regardless of the dietary P condition. Duration of naupliar phase and body length at stage CI in offspring were independent of the initial maternal P investment, implying a low P demand for early copepod development. As such, female copepods may down-regulate P investment per offspring to offset impairment of P deficiency on their fitness, and to maximize number of offspring at a given level of P availability.

An inter-basin comparison of nutrient limitation and the irradiance response of pulse-amplitude modulated (PAM) fluorescence in Lake Erie phytoplankton

This research examined the application of the maximum quantum efficiency (Fv/Fm) and relative electron transport rate versus irradiance curves (rETR) as a rapid, sensitive assessment of Lake Erie phytoplankton nutrient status. I evaluated the potential benefits of the variable fluorescence parameters by comparing these parameters with chemical and physiological nutrient status assays. I tested the hypothesis that Fv/Fm and rETR curves could diagnose nutrient status in natural lake phytoplankton and be capable of discriminating which inorganic nutrient is limited temporally and spatially. Fv/Fm was on average highest in the more eutrophic west basin (WB) and lowest in the more oligotrophic central basin (CB). According to the chemical and physiological indicators, P deficiency was most severe in the CB during summer stratification and N deficiency was strongest in the WB during isothermal conditions. Like Fv/Fm, rETR at light saturation (rETRmax) and the initial slope of the rETR versus irradiance curve (α) decreased as the severity of N and P deficiency increased. Amendment with N or P stimulated increased Fv/Fm, rETRmax, and α in N- and P-limited samples, respectively, and abolished the photoinhibition apparent in rETR curves of nutrient-limited samples. These results supported the view that the N and P deficiency assays, and corresponding variations of variable fluorescence parameters, were valid indicators of widely variable N and P deficiency in the phytoplankton, and could be used to provide a promising tool in determining phytoplankton nutrient status. Contrary to my hopes, it did not appear that rETR–irradiance curves could discriminate between N and P deficiency. Identification of the most limiting nutrient still demanded additional information beyond the variable fluorescence measurements.

Strong spatial differentiation of N and P deficiency, primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya, East Africa) and the implications for nutrient management

Summary Study of phytoplankton nutrient status, biomass, productivity and species composition was carried out between March 2005 and March 2006, along a transect between north‐eastern open Lake Victoria and the large, shallow Nyanza Gulf in order to examine how the terrestrial run‐off can influence phytoplankton community and nutrient status and determine whether nutrient management of catchment run‐off has the potential to control the algal blooms in the gulf. Hydrological and nutrient differences between the open lake and the gulf create a transition from P deficiency for phytoplankton within the gulf to nitrogen deficiency in open lake. The shallow and turbid gulf was continuously dominated by non‐nitrogen‐fixing filamentous and chroococcale colonial cyanobacteria, but seasonal stratification and deeper mixing depth in the open lake favoured diazotrophic cyanobacteria and diatoms. Seston ratios and metabolic nutrient assays indicated the gulf to be sufficiently phosphorus deficient to impose P limitation on phytoplankton growth and biomass. In contrast, the open lake is not P deficient and is more likely to experience N deficiency that favours diazotrophic cyanobacteria. Because of high turbidity in the gulf, the euphotic zone is very shallow, limiting integral primary productivity compared to the less turbid open lake; high PAR extinction may also favour Microcystis blooms in the gulf. Increased P loading into the gulf may translate to higher algal biomass, mainly of the bloom‐forming and potentially toxic cyanobacteria, and therefore, reduction in P loading into the gulf should be a management priority. However, a review of historical data indicates that the greatest change in water quality in the gulf is increased turbidity that reduces light availability and may limit algal growth more than P deficiency in years of high rainfall and river discharge.

Nearshore–offshore differences in planktonic chlorophyll and phytoplankton nutrient status after dreissenid establishment in a large shallow lake

In Lake Simcoe, a large lake in southern Ontario, Canada, with more than 50% of its surface area <15 m deep, dreissenid mussels are abundant in the extensive nearshore zone but not offshore. We hypothesized that mussel grazing would depress chlorophyll a (Chl-a) concentrations in the nearshore compared to the offshore while alleviating nearshore phosphorus (P) deficiency through nutrient regeneration. During both years of our study Chl-a concentration and other indicators of phytoplankton biomass, including particulate carbon (C), nitrogen, P, and silicon, were lower in the nearshore areas of Lake Simcoe where the exotic invader Dreissenia polymorpha was in contact with overlying epilimnetic water. In the first year of our study, grazing and associated nutrient regeneration activity seemed to reduce P deficiency in phytoplankton in the dreissenid-impacted shallow locations. In the second year, however, phytoplankton in the nearshore dreissenid-affected areas remained as strongly P deficient as phytoplankton in offshore waters physically separated from dreissenid grazing. Photoacclimation in the nearshore phytoplankton was evident in higher particulate C:Chl-a ratios and higher effective absorptive cross section of photosystem II (σPS II) throughout the stratified sampling season compared to offshore phytoplankton. A multiple linear regression utilizing the mean light intensity in the mixed layer as well as total P (TP) resulted in better predictions of Chl-a than TP alone. We conclude that, in shallow lakes where transparency is strongly impacted by dreissenid grazing, the comparison of Chl-a–TP relationships over time will require accounting for the effect of changing transparency.

Distribution of seston and nutrient concentrations in the eastern basin of Lake Erie pre- and post-dreissenid mussel invasion

Increased human population growth, reduction of phosphorus (P) loading, and the invasion of dreissenid mussels may have changed the spatial pattern and relationships between the nearshore and the offshore seston and nutrient concentrations in the eastern basin of Lake Erie over the past 30years. We compared seston characteristics, nutrient concentrations, and phytoplankton nutrient status between nearshore and offshore zones in years before (1973–1985) and after (1990–2003) the dreissenid invasion. In 1973 (the only pre-dreissenid year nearshore data was collected), chlorophyll a (chla) and nutrient concentrations were higher nearshore than offshore. In post-dreissenid years, nearshore chla concentrations became significantly lower than the offshore, while carbon (C):chla ratios became higher, which was related to mussel grazing and possibly photoacclimation. Phosphorus deficiency in the phytoplankton increased over the 30-year period, and in the post-dreissenid years was less acute in the nearshore than offshore. Mean water column irradiance became higher in the nearshore relative to the offshore in the post-dreissenid years. The nutrient changes and phytoplankton physiology were consistent with the expected effects of nutrient cycling by mussels and diminished demand by phytoplankton despite increased demand from benthic algae in the nearshore. This basin-scale study suggests that dreissenid mussel invasion can be associated with alterations in the spatial pattern of water column properties in large lakes even on open coasts with vigorous circulation and exchange.

Wind‐driven physical processes and sediment characteristics affect the distribution and nutrient limitation of nearshore phytoplankton in a stratified low‐productivity lake

Lay AbstractWind‐driven physical processes are expected to affect the spatial distribution and composition of algae in lakes and reservoirs, and to determine their access to nearshore nutrients. We used nutrient addition bioassays to detect changes in the nutrient status of phytoplankton, which indicate changes in nutrient availability in the water. We examined these effects at offshore and nearshore sites and at different times of the year, under different stratification conditions (prestratification, early and late stratification) and with different phytoplankton communities. Phytoplankton accumulated downwind, but their growth rate was usually higher at upwind than downwind sites. This suggests that the quantity and quality of algal food sources for higher trophic levels may vary in predictable but opposite ways. Wind‐driven surface waves and upwelling activity were associated with changes in phytoplankton nutrient limitation in nearshore areas, but these differences were site specific. Our results suggest that wind‐driven physical processes and sediment characteristics are both important in determining internal nutrient loading and phytoplankton nutrient limitation in nearshore areas. On windy days, nutrient limitation of offshore phytoplankton at the lake surface was always related to the conditions found upwind, suggesting rapid exchanges between nearshore and offshore areas. Wind‐driven physical processes affect the distribution and nutrient limitation of phytoplankton in lakes, and are likely to influence the efficiency of energy transfers through planktonic food webs. These wind‐driven processes should be included more specifically into food web models.

Nutrient status of phytoplankton across a trophic gradient in Lake Erie: evidence from new fluorescence methods

Variable fluorescence of chlorophyll a was measured by pulse amplitude modulated fluorometry to determine its relationship with measures of nutrient status and phytoplankton community structure in Lake Erie. In 2005, nitrogen (N) deficiency was most common in May, phosphorus (P) deficiency was most common in June, and neither were common in September. The maximum quantum yield (Fv/Fm) measured by pulse amplitude modulated fluorometry was lower in May and June than in September. The observed range of Fv/Fmincluded many values lower than previously reported in the lower Laurentian Great Lakes, while Fv/Fmvalues showed strong inverse correlations with indicators of N and (or) P deficiency. Community structure was also associated with nutrient status. Cyanobacteria were common at sites displaying N deficiency, while flagellates dominated P-deficient sites in all basins. N deficiency is surprising in a lake with generally high nitrate levels, but was supported by N debt, particulate C:N ratios and depressed Fv/Fm. Further work to characterize and compare results obtained with different variable fluorescence methodologies is desirable, but the present results support the belief that Fv/Fmcan characterize nutrient deficiency of phytoplankton community in this large lake.

MODELING THE RELEASE OF DISSOLVED ORGANIC MATTER BY PHYTOPLANKTON(1).

Three models describing dissolved organic matter (DOM) flux and phytoplankton death, each of different levels of complexity, were constructed and tested against experimental data for a cyanobacterium, a chlorophyte, two diatoms, two dinoflagellates, and two prymnesiophytes. The simplest model described only bulk carbon (C) and nitrogen (N) forms of DOM (DOMC and DOMN ) and employed a fixed relationship between phytoplankton nutrient status and DOM release and death rate. The most complex model described fractions of DOM as low molecular weight dissolved organic carbon (DOC; saccharides, low molecular weight carbohydrates [DOCs]), low molecular weight nitrogenous material (comprising C and N as DOC associated with low molecular weight compounds containing amino acids and/or nucleic acids [DOCa] and N associated with DOCa [DONa], which included dissolved free amino acids [DFAA]), and more complex materials (DOC associated with high molecular weight compounds typically requiring extracellular degradation prior to uptake or use by microbes [DOCx] and N associated with DOCx [DONx]). It also employed descriptions of DOM flux and cell death related to nutrient status and growth rates. In all instances, material lysed from dead cells contributed to the DOM pool. All three models captured the gross dynamics of the primary data (dissolved inorganic C [DIC], dissolved inorganic N [DIN], particulate organic carbon [POC], particulate organic N [PON], DOC, dissolved organic N [DON]), but there was little or no improvement of the fit with increasing model complexity. However, the simplest models tended to employ excessively high growth rates to compensate for high fixed death rates. While the proportion of newly fixed C being liberated as DOMC (DOCs plus DOCa) increased as nutrient status declined, the actual rate of release typically did not do so and often declined. The most complex model gave predictions for changes in released saccharides and DFAA in keeping with expectations. The major obstacle to future progress is the lack of suitable, mass balanced data sets for further model testing.

Phytoplankton Nutrient Status in Lake Erie in 1997

Reduction of phosphorus (P) inputs to Lake Erie since the 1970s have resulted in lower mean total phosphorus (TP) concentration in the lake and lower mean chlorophyll a concentration. In this study we examine indicators of phytoplankton nutritional status to determine whether the observed decrease in TP concentration has resulted in a strongly P deficient phytoplankton community. Phytoplankton nutrient status measurements including alkaline phosphatase activity (APA), P and nitrogen (N) debt, PO 4 turnover time and particulate carbon (C), N and P stoichiometric ratios, were made in all three basins of Lake Erie from May through October in 1997. The phytoplankton in the western basin only infrequently showed signs of nutrient deficiency over the May through October sampling season. Phytoplankton in the large central basin were moderately P deficient during the stratified season except for a period of strong deficiency in July, the time of maximum heat content and strong stratification. The eastern basin became moderately P deficient with the onset of stratification and remained moderately P deficient over the entire stratified season and no indications of extreme deficiency were observed. Compared to several other lakes in the same geoclimatic region as Lake Erie, including Lake Superior, the phytoplankton community was not strongly nutrient deficient.

Assessment of the nutrient status of phytoplankton: a comparison between conventional bioassays and nutrient-induced fluorescence transients (NIFTs)

Determining nutrient limitation in natural phytoplankton populations is problematic, given the theoretical and practical limitations of popularly used methods such as nutrient enrichment bioassays, which involve adding various nutrient mixtures to samples in containers and then measuring growth over an incubation period of several days. Laboratory studies suggest that a rapid fluorometric technique based on the concept of nutrient-induced fluorescence transients (NIFTs) may provide a direct measure of the nutrient status of natural phytoplankton populations. NIFTs were measured on laboratory cultures of Chlorella emersoniiand Oscillatoria sp., and consistent NIFT responses were recorded in nutrient depleted cultures. NIFTs and bioassays were compared using water samples from the Yarra River and Port Phillip Bay, Victoria, and from the Derwent Estuary in Tasmania. Bioassays indicated potential (Liebig) limitation by either phosphorus or nitrogen or both at every site sampled, while NIFT measurements taken on the day of collection indicated no nutrient limitation at any site at that time. After a number of days of controlled incubation, positive NIFT responses were observed in several of the water samples, although not always in response to the nutrient that the bioassay indicated was limiting. The data presented suggest that conventional bioassays do not measure true, in situ nutrient limitation, and instead only provide information about potential limitation (Liebig limitation) in the absence of other limiting factors. © 2004 Elsevier Ltd. All rights reserved.

Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary

Summary1. As supported by field data, turbidity recorded by light scattering sensors could reliably be converted into concentration of suspended particulate matter (SPM) and coefficient of vertical light attenuation (Kd) in Lake Balaton.2. Autocorrelation analysis revealed that proper determination of SPM concentration and Kd required daily sampling. To approximate daily rate of resuspension, 15 min or more frequent measurements were needed. Thus, routine monitoring provides very little insight into environmental variability of shallow lakes as habitats for phytoplankton.3. The internal P load was estimated from daily rate of resuspension and P desorption capacity of sediments. The latter was assumed to be proportionate to the potentially mobile inorganic P content of SPM. A comparison with net primary production and nutrient status of phytoplankton showed that the proposed method of estimating time series of internal P load captured seasonal trends.4. The daily rate of resuspension was high whereas that of internal P load was low in Lake Balaton relative to other shallow lakes. The latter reflects favourable behaviour of the calcite‐rich sediments. As a consequence, carrying capacity of Basin 1 of Lake Balaton was P‐determined.5. The timing of external and internal loads was radically different. While the former showed mostly seasonal changes, large pulses characterised the latter. As a consequence, internal load may supply more P to phytoplankton growth during the critical summer months than external load. However, the relative importance of these sources may show substantial interannual variability.6. Large resuspension events often followed each other during periods of 10–15 days. It has been shown that disturbances in this frequency range are of key importance in maintaining the diversity of phytoplankton. We propose that resuspension can be perceived not only as a disturbance factor but also as a factor that periodically relaxes nutrient stress. The former feature may dominate the instantaneous effect, whereas the latter may determine the persistent effect of resuspension on succession of phytoplankton.

Effects of ultraviolet‐B radiation and vertical mixing on nitrogen uptake by a natural planktonic community shifting from nitrate to silicic acid deficiency

We investigated the combined effects of mixing regimes and ultraviolet‐B radiation (UVBR: 280–320 nm) on the uptake of nitrate, ammonium, and urea by an estuarine phytoplankton community during a 10‐d mesocosm experiment in July 1997. The experiment was conducted in eight mesocosms (1,500 liters each) subjected to varying conditions of UVBR and mixing: (1) natural UVBR and fast mixing, (2) enhanced UVBR and fast mixing, (3) natural UVBR and slow mixing, and (4) enhanced UVBR and slow mixing. The study was carried out during a typical midsummer period of low nutrient concentrations. Throughout the experiment, the phytoplankton community was dominated by a mixed community composed mainly of centric diatoms, cyanobacteria, and prymnesiophytes, with larger algal cells (&gt;5 µm) representing &gt;70% of the total chlorophyll a biomass. The phytoplankton nutrient status changed over the experiment. Algal cells were nitrate‐deficient until the occurrence of a nitrification event on day 7, and then became silicic acid‐deficient. Mesocosms with different mixing rates showed significant differences in ambient nutrient concentrations, nitrogen uptake rates, and phytoplankton species composition. In general, no significant difference was detected between the N transport rates from samples incubated close to the surface and close to the bottom of the mesocosms, and no difference was detected between the UVBR treatments on all the other studied biological and chemical variables measured during the experiment. It is only under the slow mixing regime that the enhanced UVBR treatments significantly depress both the transport and the chlorophyll a—specific transport rates of dissolved organic nitrogen (urea). The UVBR effects observed only in the slow mixing regime could be a consequence of an inherent difference in the uptake capacity of the different communities under the two mixing regimes. Both nutrient conditions and mixing regime influenced the natural phytoplankton physiology and composition toward a community more sensitive to UVBR enhancement under a slower mixing regime.

Physiological indicators of nutrient deficiency in phytoplankton in southern Chilean lakes

We assessed the nutrient status of phytoplankton in 28 lakes in southern Chile using two types of physiological indicators: specific alkaline phosphatase activity, and the elemental composition (carbon, nitrogen, and phosphorus) of seston. Alkaline phosphatase activity ranged from 0.001 to 0.11 μmol P μg chl−1 h−1, with P-deficiency indicated in about one-half the study lakes. C:N ranged from 3.9 to 24, C:P ranged from 86 to 919, and N:P ranged from 8.7 to 99. C:P and N:P ratios greater than the Redfield ratio were common, suggesting P deficiency in many of the lakes. C:N ratios were not generally indicative of N deficiency. Previous studies have suggested N may be the primary limiting nutrient in southern Chilean lakes, but our results indicate that P should not be discounted as a limiting nutrient.

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of 14C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of 14C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.

Phytoplankton nutrient status and mean water column irradiance in Lakes Malawi and Superior

Abstract Phytoplankton growth in Lake Malawi was moderately nitrogen and phosphorus-deficient according to indicators of phytoplankton nutrient status (particulate C:N, C:P, N:P composition ratios, nitrogen and phosphorus debt assays) and occasionally light-deficient during the period of deepest mixing (July and August). Phytoplankton in Lake Superior was light-deficient during most of the year because of the deeply mixed water column. However, during the stratified period when the mean water column irradiance increased, phytoplankton in Lake Superior became severely phosphorus-deficient according to the same nutrient status indicators used in Lake Malawi as well as alkaline phosphatase activity. Specific rates of carbon uptake normalized to particulate carbon, calculated from photosynthesis at optimum light, were on average three times greater in Lake Malawi than in Lake Superior. We calculated that nitrogen and phosphorus inputs from rivers and precipitation supplied &amp;lt; 15% of the demand for these elements due to photosynthesis for both Lake Malawi and Lake Superior and could not explain the observed difference in phytoplankton nutrient status or specific rate of carbon uptake normalized to particulate carbon. The ratio of nitrogen to phosphorus supplied in Lake Malawi is lower and closer to Redfield ratios than that in Lake Superior. We speculate that the more balanced supply ratio of these nutrients, combined with higher rates of internal regeneration in the warmer deeper mixed layer of Lake Malawi, result in phytoplankton that is less nutrient-deficient and has higher specific rates of carbon uptake normalized to particulate carbon than Lake Superior.

Phytoplankton nutrient status and mean water column irradiance in Lakes Malawi and Superior

Phytoplankton growth in Lake Malawi was moderately nitrogen and phosphorus-deficient according to indicators of phytoplankton nutrient status (particulate C:N, C:P, N:P composition ratios, nitrogen and phosphorus debt assays) and occasionally light-deficient during the period of deepest mixing (July and August). Phytoplankton in Lake Superior was light-deficient during most of the year because of the deeply mixed water column. However, during the stratified period when the mean water column irradiance increased, phytoplankton in Lake Superior became severely phosphorus-deficient according to the same nutrient status indicators used in Lake Malawi as well as alkaline phosphatase activity. Specific rates of carbon uptake normalized to particulate carbon, calculated from photosynthesis at optimum light, were on average three times greater in Lake Malawi than in Lake Superior. We calculated that nitrogen and phosphorus inputs from rivers and precipitation supplied <15% of the demand for these elements due to photosynthesis for both Lake Malawi and Lake Superior and could not explain the observed difference in phytoplankton nutrient status or specific rate of carbon uptake normalized to particulate carbon. The ratio of nitrogen to phosphorus supplied in Lake Malawi is lower and closer to Redfield ratios than that in Lake Superior. We speculate that the more balanced supply ratio of these nutrients, combined with higher rates of internal regeneration in the warmer deeper mixed layer of Lake Malawi, result in phytoplankton that is less nutrient-deficient and has higher specific rates of carbon uptake normalized to particulate carbon than Lake Superior.

Effects of Lake Size on Phytoplankton Nutrient Status

Phytoplankton nutrient status measurements (C/P, C/N, C/chlorophyll, N/P, alkaline phosphatase activity, and N debt) were measured for 6 yr in seven remote Canadian Shield lakes. Lakes Nipigon and Superior were also studied for 2 yr. These lakes varied in surface area from 29 to 8.223 × 10 ha, they all stratified fully during the summer and had water renewal times &gt; 5 yr. All lakes were severely P deficient; however, the large lakes (&gt; 2000 ha) were consistently less P deficient than small lakes. A growth-rate indicator (photosynthesis normalized to particulate C) agreed with nutrient status indicators, in that small lakes had lower rates than large lakes. Total P was a good predictor of chlorophyll, but factors related to lake size (temperature and mixed depth) were equally good or better predictors of nutrient status. Decreasing mean water column light intensity could not explain the lower P deficiency of large lakes. The deeper, more energetic mixed layers in large lakes apparently cause P to be recycled more efficiently. Extrapolation of observations or experimental results from small to large lakes requires recognition that phytoplankton in large lakes are less nutrient deficient and may have higher growth rates.

Recent Changes of N2-Fixation in Lake 227 in Response to Reduction of the N:P Loading Ratio

Continuous artificial additions of nitrogen (N) and phosphorus (P) to Lake 227 at a loading ratio of 11:1 (atom: atom; 5:1 by mass) from 1975 to 1989, combined with natural inputs, resulted in variable mean annual total N and total P concentrations and changes in phytoplankton species composition. Summer phytoplankton occasionally shifted from N2-fixing cyanobacteria to non-N2-fixing forms. In 1990, additions of N fertilizer were stopped in order to test the hypothesis that low N:P loading rates cause selection for heterocystous cyanobacteria and stimulate N2-fixation. Five years of phytoplankton nutrient-status measurements, together with estimates of N2-fixation before and after lowering the N:P loading ratio, verified the importance of this ratio in producing blooms of N2-fixing cyanobacteria. Nitrogen input via biological N2fixation increased, and summer dominance by heterocystous cyanobacteria returned as a consequence of lowering the N:P ratio of external inputs. Seasonally, N return from the epilimnetic sediments increased after a period of high planktonic N2-fixation rates. This regenerated N reduced the abundance of heterocystous cyanobacteria and lowered late summer N2-fixation rates. Over several years, preferential regeneration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loading ratio.

Carbon fixation into lipid in small freshwater lakes

Algal lipid is a critical food resource for zooplankton, a solvent for lipophilic contaminants, and a useful indicator of phytoplankton nutrient status in culture. Seasonal patterns in lipid production and the photosynthetic parameters describing lipid production were determined for three mesotrophic to oligotrophic, headwater lakes. The mean lipid fraction of C fixation (LFCF) was 15.8% (range 8.5-22.5%) measured bimonthly from 2 May to 2 1 October 1988. Periods of highest nutrient stress were associated with the lowest LFCF, contradicting findings from algal culture experiments and suggesting that nutrient deficiency does not govern lipid synthesis in natural aquatic ecosystems. LFCF was related instead to temperature and daylength, suggesting that it is related to changes in the physical environment.