The dilemma of controlling cultural eutrophication of lakes

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.     

Fishery-aspects of eutrophication

Summary Eutrophication influences among others food supply, prey catchability, reproduction success, growth and mortality of fish. The first stages of eutrophication are favourable for many fish species. Proceeding eutrophication interferes so strongly with the environment that fish is brought in a vulnerable position: vegetation, which is a prerequisite for a number of species, disappears; oxygen depletion near the bottom affects fish food organisms; turbidity hampers catchability of preyfish and decomposition of dead phytoplankton or vegegation may especially at the end of the summer result in lethal oxygen contents. Hence at higher eutrophic levels the fish population surpasses the optimum and decreases again. These optima vary with the species; for the most important species the sequence from oligotrophic to hypertrophic is: 1. Coregonids and Salmondis, 2. pike, 3. roach and perch, 4. pikeperch, 5. bream. Hence the fish populations of very turbid hypertrophic waters are dominated by bream and pikeperch. This population will furthermore usually consist of smaller quantities of eel, smelt, ruffe and white bream. The total fish population in such a hypertrophic water is considerably larger than in a oligotrophic or mesotrophic lake. The vulnerability of the fish community, however, has increased too. In the hypertrophic situation the oxygen supply is the weak spot and every interfering negative influence, as for example sewage discharge, may have disastrous consequences. The many severe fish mortalities prove this to be a real danger.     

Seagrasses and eutrophication

This review summarizes the historic, correlative field evidence and experimental research that implicate cultural eutrophication as a major cause of seagrass disappearance. We summarize the underlying physiological responses of seagrass species, the potential utility of various parameters as indicators of nutrient enrichment in seagrasses, the relatively sparse available information about environmental conditions that exacerbate eutrophication effects, and the better known array of indirect stressors imposed by nutrient over-enrichment that influence seagrass growth and survival. Seagrass recovery following nutrient reductions is examined, as well as the status of modeling efforts to predict seagrass response to changing nutrient regimes.The most common mechanism invoked or demonstrated for seagrass decline under nutrient over-enrichment is light reduction through stimulation of high-biomass algal overgrowth as epiphytes and macroalgae in shallow coastal areas, and as phytoplankton in deeper coastal waters. Direct physiological responses such as ammonium toxicity and water-column nitrate inhibition through internal carbon limitation may also contribute. Seagrass decline under nutrient enrichment appears to involve indirect and feedback mechanisms, and is manifested as sudden shifts in seagrass abundance rather than continuous, gradual changes in parallel with rates of increased nutrient additions. Depending on the species, interactions of high salinity, high temperature, and low light have been shown to exacerbate the adverse effects of nutrient over-enrichment. An array of indirect effects of nutrient enrichment can accelerate seagrass disappearance, including sediment re-suspension from seagrass loss, increased system respiration and resulting oxygen stress, depressed advective water exchange from thick macroalgal growth, biogeochemical alterations such as sediment anoxia with increased hydrogen sulfide concentrations, and internal nutrient loading via enhanced nutrient fluxes from sediments to the overlying water. Indirect effects on trophic structure can also be critically important, for example, the loss of herbivores, through increased hypoxia/anoxia and other habitat shifts, that would have acted as “ecological engineers” in promoting seagrass survival by controlling algal overgrowth; and shifts favoring exotic grazers that out-compete seagrasses for space. Evidence suggests that natural seagrass population shifts are disrupted, slowed or indefinitely blocked by cultural eutrophication, and there are relatively few known examples of seagrass meadow recovery following nutrient reductions.Reliable biomarkers as early indicators of nutrient over-enriched seagrass meadows would benefit coastal resource managers in improving protective measures. Seagrasses can be considered as “long-term" integrators (days to weeks) of nutrient availability, especially through analyses of their tissue content, and of activities of enzymes such as nitrate reductase and alkaline phosphatase. The ratio of leaf nitrogen content to leaf mass has also shown promise as a “nutrient pollution indicator” for the seagrass Zostera marina, with potential application to other species. In modeling efforts, seagrass response to nutrient loading has proven difficult to quantify beyond localized areas because long-term data consistent in quality are generally lacking, and high inter-annual variability in abundance and productivity depending upon stochastic meteorological and hydrographic conditions.Efforts to protect remaining seagrass meadows from damage and loss under eutrophication, within countries and across regions, are generally lacking or weak and ineffective. Research needs to further understand about seagrasses and eutrophication should emphasize experimental studies to assess the response of a wider range of species to chronic, low-level as well as acute, pulsed nutrient enrichment. These experiments should be conducted in the field or in large-scale mesocosms following appropriate acclimation, and should emphasize factor interactions (N, P, C; turbidity; temperature; herbivory) to more closely simulate reality in seagrass ecosystems. They should scale up to address processes that occur over larger scales, including food-web dynamics that involve highly mobile predators and herbivores. Without any further research, however, one point is presently very clear: Concerted local and national actions, thus far mostly lacking, are needed worldwide to protect remaining seagrass meadows from accelerating cultural eutrophication in rapidly urbanizing coastal zones.     

Ecological expected utility and the mythical neural code

Neural spikes are an evolutionarily ancient innovation that remains nature’s unique mechanism for rapid, long distance information transfer. It is now known that neural spikes sub serve a wide variety of functions and essentially all of the basic questions about the communication role of spikes have been answered. Current efforts focus on the neural communication of probabilities and utility values involved in decision making. Significant progress is being made, but many framing issues remain. One basic problem is that the metaphor of a neural code suggests a communication network rather than a recurrent computational system like the real brain. We propose studying the various manifestations of neural spike signaling as adaptations that optimize a utility function called ecological expected utility.     

Lake eutrophication and community structure

The role of external and internal phosphorus loading in the lake eutrophication was estimated according to published data and our own. The role of plankton and benthos animals has been determined in phosphorus excretion and the formation of internal nutrient load. It is shown that the term “eutrophication” is valid both in the anthropogenic and natural increases in the trophic status of the water body.     

The possibilities of improving catchable fish stocks in lakes undergoing eutrophication

The area of Mazurian Lakeland is the biggest lake concentration in Middle Europe. Its lakes are subjected to various man-made impacts and disturbances, resulting in many changes in the aquatic environment and the fish stocks. Most frequently these changes are typical of the process of accelerated eutrophication, and are connected with undesirable succession in the fish stocks. Generally, predatory species, which naturally regulate the stock and maintaiil its balance, disappear from the environment, as do other valuable fish species, e.g. coregonids. At the same time food resources (weed fishes, plankton) for those species develop abundantly.
Assessment of the management of lakes by one of the State Lake Fish Farms located in Mazurian Lakeland is presented against a background of various man-made influences on the aquatic ecosystems in this region. The State Fish Farm under study manages 56 lakes of total area over 5800 ha. For each of these lakes detailed records were available of the commercial catches of particular species and their artificial stockings over a period of 31 years. These, their trends and inter-relationships have been analyzed. Artificial stockings represented one of the best methods of counteracting adverse changes taking place in the fish stocks, with a simultaneous utilization of the productive potential of the lakes. Against this background, forecasts are made of the expected changes, and basic approaches to the proper management of the fish stocks are suggested.     

Biological monitoring of eutrophication in rivers

There is an increasing awareness of the need to assess the impact of nutrient enrichment on river ecosystems separately from the impacts of organic effluents. A range of methods have been proposed and some have moved from the development stage to practical use by water management organizations. The methods can be applied to broad surveys or provide baseline information to assess possible future change. In the latter case it is recommended that several different methods are used, especially where it is important to get reliable information on the long-term impact of improvements in effluent quality. Estimates of biomass measured as chlorophyll a have often been used for phytoplankton and sometimes also for benthic communities. However, a lot of care is needed in applying this method, because of the range of factors besides nutrient concentration which can influence values. Approaches based on the whole community have been developed by a number of research groups, usually involving semiquantitative estimates of abundance. There has also been a rapid increase in the use of indices based on the relative proportions of epilithic diatom species. The methodologies used by a number of research groups in Europe are broadly similar, making it possible to compare results between different regions. The development of indices based on macrophyte floristic composition in relation to river nutrient status is also under development, especially in France and UK. However, interpretation of the results is complicated where long-term changes are taking place in nutrient concentrations in the water, because of the varying contributions of sediment and water to different species of rooted plant. Bioassays can be especially helpful where it is desired to establish whether either N or P is limiting for a population of community.     

Internal eutrophication of restored peatland stream: The role of bed sediments

The improvement of site hydrology is a major determinant of the success or failure of wetland restoration. Unfortunately, the influence of new hydrological conditions on wetland chemistry and nutrient cycling is often ignored or simply not monitored. This study assessed how changes in the composition of stream sediments that occurred after peatland stream restoration affected the pool and bioavailability of phosphorus (P) stored on the stream floor. The studied watercourses were located in the Narew River valley, NE Poland. They were restored in 2002 by excavating channels about 4-6 m wide and 1-1.5 m deep. Channels were cut through a peat layer to basal sands that underlay the organic deposits. Due to fallacious assumption about the high stability of peat stream banks, the banks were not modelled or protected by any technical measures to eliminate potential slump and erosion.Within a few years of the completion of the restoration project, peat bank failures led to the substantial deposition of organic material onto the floor of the newly created water-bodies. This resulted in an increase in the mean total phosphorus (TP) concentration in the sediment from 19 μmol g⁻¹ to more than 35 μmol g⁻¹ as well as an increase in both the concentration and pool of potentially mobile P fractions. A potential for the release of P was confirmed by the change in the concentration of P fractions that has been recorded over the summer period. Between June and October 2008 their content in the top 1-cm layer diminished from 134.1 mmol m⁻² to 100.6 mmol m⁻², implying an average net release of P of about 0.3 mmol m⁻² day⁻¹.This suggests that examined sediments primarily act as a highly dynamic transformer system, being the sink for particulate organic and mineral forms of P and serving as the net source of bioavailable (soluble reactive phosphorus) SRP.