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The management of eutrophication has been impeded by reliance on short-term experimental additions of nutrients to bottles and mesocosms. These measures of proximate nutrient limitation fail to account for the gradual changes in biogeochemical nutrient cycles and nutrient fluxes from sediments, and succession of communities that are important components of whole-ecosystem responses. Erroneous assumptions about ecosystem processes and lack of accounting for hysteresis during lake recovery have further confused management of eutrophication. I conclude that long-term, whole-ecosystem experiments and case histories of lake recovery provide the only reliable evidence for policies to reduce eutrophication. The only method that has had proven success in reducing the eutrophication of lakes is reducing input of phosphorus. There are no case histories or long-term ecosystem-scale experiments to support recent claims that to reduce eutrophication of lakes, nitrogen must be controlled instead of or in addition to phosphorus. Before expensive policies to reduce nitrogen input are implemented, they require ecosystem-scale verification. The recent claim that the ‘phosphorus paradigm’ for recovering lakes from eutrophication has been ‘eroded’ has no basis. Instead, the case for phosphorus control has been strengthened by numerous case histories and large-scale experiments spanning several decades.

Two small, oligotrophic lakes at the IISD-Experimental Lakes Area in northwestern Ontario, Canada were fertilized weekly with only phosphorus (P) in the summer and early fall of 2019. The P fertilization rates were high enough (13.3 µ g l −1 added weekly) to produce dense, month-long blooms of N 2 -fixing Dolichospermum species in both lakes within 9–12 weeks after fertilization began, turning them visibly green without the addition of nitrogen. P-only fertilization increased average seasonal chlorophyll a concentrations and cyanobacteria biomass well above the pre-fertilization levels of 2017 and 2018. Nitrogen (N) content in the epilimnion of thermally stratified Lake 304 and the water column of shallow Lake 303 doubled and P storage in the water column temporarily increased during the blooms. These whole-lake fertilization experiments demonstrate that large cyanobacteria blooms can develop rapidly under high P loading without anthropogenic N inputs, suggesting that aggressive N control programs are unlikely to prevent bloom formation and that P controls should remain the cornerstone for cyanobacteria management.

We reviewed 24 sockeye salmon (Oncorhynchus nerka) nursery lake experiments that involved whole-lake fertilization with appropriate treatment and control years. We found that: 21 of 21 studies showed that fertilization was associated with increased chlorophyll a concentrations, 16 of 16 showed increased zooplankton biomasses, 16 of 16 demonstrated increased average smolt weights, and 11 of 13 showed increased smolt biomasses. Studies involving assessments of egg-to-smolt survival were rare, but all (4 of 4) showed increased survival rates. Studies involving increased smolt-to-adult survival (i.e., marine survival) were even rarer, but all (3 of 3) showed that lake fertilization and increased smolt size were associated with increased marine survival. Several fertilization studies reported problems, and some offered solutions. For instance, when whole-lake fertilization stimulated the growth of blue-green algae, fertilizer with higher nitrogen to phosphorus ratios was used to control the problem. Conversely, when high nitrogen to phosphorus ratios were associated with blooms of ungrazable diatoms, notably Rhizosolenia eriensis, reduced nitrate concentrations were recommended. To date, solutions designed to constrain the growth of both blue-green algae and Rhizosolenia blooms remain elusive. Some studies showed that when both mysids (large invertebrate planktivores) and juvenile sockeye inhabit the same lake, sockeye suffer from a competitive disadvantage and mysids consume 80–90% of the available zooplanktonic food production. Similarly, a small number of studies demonstrated that competition from sticklebacks (Gasterosteus aculeatus) adversely affected sockeye growth rates, and although the problem remains unresolved, ongoing work in lakes containing kokanee (O. nerka), suggests that stocked cutthroat trout (Salmo clarki) may be capable of controlling stickleback densities through predation. Despite all of these difficulties, in almost all cases, when lakes were fertilized with various mixtures of inorganic nitrogen and phosphorus, pelagic food web bottom-up control was strong enough and predictable enough to ensure that sockeye smolt biomass increased. We conclude that sockeye nursery lake fertilization is a technique that can contribute usefully to both the enhancement and conservation of sockeye salmon populations. Key words: sockeye salmon, lake fertilization, bottom-up, aquatic food web.

Ecosystem ecologists often face the challenge of predicting long-term consequences of perturbations such as nutrient enrichment from shorter term experiments. In such experiments, the spatial and temporal scale at which data are analyzed can directly impact extrapolations to larger scales. Here, we assess the level of data resolution required to answer qualitative and quantitative questions about primary production in a set of lake ecosystems. We tested the ability of 40 models containing variable levels of spatial and temporal complexity to (1) fit the relationship between light and primary productivity in a set of 14 British Columbia (Canada) lakes that were part of a large-scale fertilization experiment, and (2) predict annual primary production. The experimental data had previously been averaged across fertilization treatments for analysis, effectively ignoring spatial and temporal variation. In the limnological literature, data from whole-lake experiments are often analyzed for each lake to account for among-lake differences, or analyzed for each lake in each year to account for both lake effect and interannual variation. Using an information-theoretic approach, we tested these three models (\"fertilization status only,\" \"lake only,\" and \"lake and year\") against models that included less and more spatial and temporal partitioning of the data. We fit a Monod function to light and primary productivity at 40 spatial and temporal levels of data resolution and ranked the model fits using the second-order Akaike Information Criterion (AICc). The \"fertilization status only\" model ranked 37th out of 40 models tested, the \"lake only\" model ranked 27th, and the \"lake and year\" model ranked 25th. The top-ranked model partitioned the data by lake, year, fertilization status, season, and sampling station, and fit the data substantially better than other models. We then calculated primary production with independent light data and the fitted model parameters to compare the top three models from our AICcranking and the \"lake only,\" \"lake and year,\" and \"fertilization status only\" models. The predicted production differed depending on the model, but all models predicted higher net primary productivity in fertilized lakes. Model selection is therefore important for quantitative predictions, but not necessarily for qualitative assessment of nutrient limitation.

We investigated whether pelagic food web dynamics, expressed through a trophic triangle, could influence the potential success of whole-lake fertilization to enhance juvenile sockeye salmon growth. Muriel Lake (145 ha), located on Vancouver Island, was fertilized during 1984 with no apparent effect on juvenile sockeye growth. Unlike most sockeye nursery lakes, Muriel Lake contains a substantial population of the invertebrate zooplanktivore Neomysis mercedis. We hypothesized that competition for zooplankton prey between Neomysis and planktivorous fish (juvenile sockeye, threespine stickleback) could counteract beneficial effects of either natural or fertilizer-induced increases in food for fish. To test this, we assessed (1985–1986) biomass, production, and consumption of planktivorous mysids and fish and then used bioenergetics models to quantify potential competitive interactions. Our analysis suggested that N. mercedis consumed 7–8% of the zooplankton standing stock per day, while planktivorous fish consumed <1.0%·d−1. Although mysids were the main consumers of zooplankton, mysids were also consumed by fish. Late in 1986, an increase in mysid consumption by large, 1+ juvenile sockeye precipitated substantial declines in Neomysis biomass. Although this event came too late to reduce mysid competition with sockeye fry in Muriel Lake in 1986, it did highlight the potential importance of trophic triangles in pelagic food webs. We suggest that mysids may be held in check by juvenile sockeye when exogenous recruitment events result in high sockeye biomass. By contrast, recruitment failures and low sockeye biomass promote increases in Neomysis populations, which then control zooplankton communities such that sockeye gain little benefit from either natural or fertilizer-induced increases of zooplankton. For food web ecologists, the implication is that trophic triangles might produce alternate stable states that are mediated by external factors. For fisheries managers, the implication is that lakes containing mysids should only be fertilized when sockeye densities are high or mysid densities are low.

We investigated whether pelagic food web dynamics, expressed through a trophic triangle, could influence the potential success of whole-lake fertilization to enhance juvenile sockeye salmon growth. Muriel Lake (145 ha), located on Vancouver Island, was fertilized during 1984 with no apparent effect on juvenile sockeye growth. Unlike most sockeye nursery lakes, Muriel Lake contains a substantial population of the invertebrate zooplanktivore Neomysis mercedis. We hypothesized that competition for zooplankton prey between Neomysis and planktivorous fish (juvenile sockeye, threespine stickleback) could counteract beneficial effects of either natural or fertilizer-induced increases in food for fish. To test this, we assessed (1985-1986) biomass, production, and consumption of planktivorous mysids and fish and then used bioenergetics models to quantify potential competitive interactions. Our analysis suggested that N. mercedis consumed 7-8% of the zooplankton standing stock per day, while planktivorous fish consumed < 1.0%·d- 1 . Although mysids were the main consumers of zooplankton, mysids were also consumed by fish. Late in 1986, an increase in mysid consumption by large, 1+ juvenile sockeye precipitated substantial declines in Neomysis biomass. Although this event came too late to reduce mysid competition with sockeye fry in Muriel Lake in 1986, it did highlight the potential importance of trophic triangles in pelagic food webs. We suggest that mysids may be held in check by juvenile sockeye when exogenous recruitment events result in high sockeye biomass. By contrast, recruitment failures and low sockeye biomass promote increases in Neomysis populations, which then control zooplankton communities such that sockeye gain little benefit from either natural or fertilizer-induced increases of zooplankton. For food web ecologists, the implication is that trophic triangles might produce alternate stable states that are mediated by external factors. For fisheries managers, the implication is that lakes containing mysids should only be fertilized when sockeye densities are high or mysid densities are low.

Time-course experiments of phosphate uptake by size-fractionated phytoplankton were conducted in oligotrophic Kennedy and Sproat Lakes. The objective was to determine if large phytoplankton obtained more phosphate than smaller cells, when the nutrient was present at higher concentrations. Studies at Kennedy Lake revealed that uptake rates in the 0.2-3.0 μm fraction were very sensitive to the time they were exposed to elevated concentrations; rates determined over the 60-120 min interval were less than 30% of those recorded over the 0-60 min interval. In contrast, there was little difference in uptake rates over these intervals for cells > 3.0 μm. At Sproat Lake phosphate incorporation into the two size fractions was followed after the aerial fertilization of the lake with inorganic nutrients. Following nutrient addition the proportion of phosphate entering the > 3.0 μm size fraction increased from ca. 35% to ca. 85%. Despite these observations, it is doubtful that larger cells are able to sequester enough phosphate from pulses to realize the same specific growth rates as their smaller counterparts.

Fertilization of a small lake with ammonium chloride for four years as part of a eutrophication experiment caused it to acidify to pH values as low as 4.6. Implications for acidification of lakes via precipitation polluted with ammonium compounds are discussed. When phosphate was supplied with the ammonium, biological nitrogen uptake, apparently by phytoplankton, was the main mechanism causing acidification. When ammonium was applied without phosphate, it accumulated to high concentrations in solution, after which nitrification caused rapid acidification. In both cases, the whole-lake efficiency of acidification was low, averaging about 13% of the potential acidification of supplied ammonium chloride (Table 2). Subsequent application of phosphate plus sodium nitrate for two years caused the pH of the lake to increase. The efficiency of alkalinization was higher than for acidification, averaging 69% of the potential alkalinization of the supplied sodium nitrate.

The relationship between crop fertilization and non-point source (NPS) pollution is a hot topic of research on agricultural NPS pollution. In this paper, the upper reaches of Erhai Lake Basin were taken as the research area. Based on rice-broad bean and maize-potato rotation, the content of chemical oxygen demand (COD), total phosphorus (TP), ammonia nitrogen (AN), nitrate nitrogen (NO3-N), and total nitrogen (TN) in water samples from experiment field plots under normal fertilization, 60% normal fertilization and 30% normal fertilization were measured, and the response relationship between fertilization levels and the values of the indicators was explored by using the method of split-plot experiments and statistical analysis. The experiment results are as follows: (1) The fertilization level of different crops had a significant (P < 0.01) effect on agricultural NPS pollution, and the effect on TP was the strongest (R2 = 0.550; P < 0.01). COD has the second highest impact. (3)The content of AN and NO3-N measured from field plots of maize and broad bean had no significant relationship with the fertilization level, but the content of nitrogen measured from plots of potato presented a downward trend with the increased amount of fertilizer. (4) Overall, the amount of fertilizer positively affected agricultural NPS pollution, and there was a correlation between the indicators. This study can provides new ideas for the evaluation and delineation of the spatial pattern of source-sink risk of NPS pollution in the basin, and puts forth new methods for the management of NPS pollution in the basins of low-latitude plateau fractured lakes similar to the Erhai Lake Basin.

Historically, warm periods enhanced the Asian summer monsoon—increased rainfall brought additional nutrients to freshwater ecosystems and increased production. However, anthropogenic aerosols have weakened the monsoon and altered lake ecosystems. Anthropogenic aerosol increases over the past few decades have weakened the Asian summer monsoon 1 , 2 , 3 with potentially far-reaching socio-economic and ecological repercussions. However, it is unknown how these changes will affect freshwater ecosystems that are important to densely populated regions of Asia. High-resolution diatom records and other proxy data archived in lake sediment cores from the Chinese Loess Plateau allow the comparison of summer monsoon intensity, lake trophic status and aquatic ecosystem responses during warming periods over the past two millennia. Here we show that an abrupt shift towards eutrophic limnological conditions coincided with historical warming episodes 4 , 5 , marked by increased wind intensity and summer monsoon rainfall leading to phosphorus-laden soil erosion and natural lake fertilization. In contrast, aerosol-affected Anthropocene warming catalysed a marked weakening in summer monsoon intensity leading to decreases in soil erosion and lake mixing. The recent warm period triggered a strikingly different aquatic ecosystem response with a limnological regime shift marked by turnover in diatom species composition now dominated by oligotrophic taxa, consistent with reductions in nutrient fertilization, reduced ice cover and increased thermal stratification 6 . Anthropogenic aerosols have altered climate–monsoon dynamics that are unparalleled in the past ∼2,000 years, ushering in a new ecological state.