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      Impacts of fish on phosphorus budget dynamics of some SA reservoirs: Evaluating prospects of 'bottom up' phosphorus reduction in eutrophic systems through fish removal (biomanipulation)

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          Abstract

          Data on fish standing stocks in 7 South African reservoirs were used to assess prospects of reducing in-lake amounts of total phosphorus (TP) through remedial biomanipulation - the removal of fish to deplete internal stocks of biomass-incorporated TP and especially to restrict enhancement of TP availability through internal 'bottom up' recycling by fish. Literature-derived conversion functions were used to estimate the quantity of TP stored in fish biomass, recycled by fish through excretion, and released from bottom sediments through carp and catfish bioturbation. This provided a quasi mass-balance assessment of these contributory influences of fish on TP budgets of reservoirs ranging from mesotrophy to hypertrophy in trophic status (annual mean TP levels of 0.04-0.51 mg/l). Absolute contributions of fish were inevitably related directly to reservoir-specific fish stock abundance, both total-fish and coarse-fish biomass levels which increased with trophic status, generating parallel absolute increases in TP sinks and internal TP loading fluxes. On overall average, total fish stock sequestered 2.2 kg TP/ha in biomass, recycled 13.8 kg TP/ha/yr through excretion, and mobilized 8.0 kg TP/ha/yr through sediment bioturbation. Average values relative to external loadings in 5 reservoirs amounted to 3.8% (biomass), 22.8% (excretion) and 11.8% (bioturbation), totalling 38.4%. Most pertinently, the relative importance of fish in reservoir TP budgets declined progressively with rising trophic status, with corresponding averages less than half (1.4, 8.7 and 5.4%, total = 15.4%) in 3 hypertrophic reservoirs (> 0.10 mg TP/ℓ). While total fish eradication plausibly reduces average internal phosphorus by some 40% relative to external load, the corresponding average reduction in hypertrophic reservoirs in greatest need of nutrient reduction is far less (~ 15%). 'Bottom-up' bioremediation accordingly offers little help in the management of nutrient-enriched reservoirs, and is essentially futile where high external nutrient loading persists.

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          Eutrophication science: where do we go from here?

          Cultural eutrophication has become the primary water quality issue for most of the freshwater and coastal marine ecosystems in the world. However, despite extensive research during the past four to five decades, many key questions in eutrophication science remain unanswered. Much is yet to be understood concerning the interactions that can occur between nutrients and ecosystem stability: whether they are stable or not, alternate states pose important complexities for the management of aquatic resources. Evidence is also mounting rapidly that nutrients strongly influence the fate and effects of other non-nutrient contaminants, including pathogens. In addition, it will be important to resolve ongoing debates about the optimal design of nutrient loading controls as a water quality management strategy for estuarine and coastal marine ecosystems.
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            Nutrient Cycling by Animals in Freshwater Ecosystems

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              Limnology: Lake and river ecosystems

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                Author and article information

                Contributors
                Role: ND
                Role: ND
                Journal
                wsa
                Water SA
                Water SA
                Water Research Commission (WRC) (Pretoria )
                0378-4738
                July 2015
                : 41
                : 4
                : 432-440
                Affiliations
                [1 ] University of KwaZulu-Natal South Africa
                [2 ] DH Environmental Consulting (Pty) Ltd South Africa
                Article
                S1816-79502015000400001
                10.4314/WSA.V41I4.01
                72654661-2067-4d44-9ee2-3a6eb28a9146

                http://creativecommons.org/licenses/by/4.0/

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                Product

                SciELO South Africa

                Self URI (journal page): http://www.scielo.org.za/scielo.php?script=sci_serial&pid=1816-7950&lng=en
                Categories
                Water Resources

                Oceanography & Hydrology
                biomanipulation,biomass sinks,bioturbation,eutrophication management,excretion,fish,phosphorus,recycling,reservoir ecosystems

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