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)

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.

Modelling the impact of sediment management on the trophic state of a tropical reservoir with high water storage variations

“…Additionally, the two-layers phosphorus budget model proposed by Chapra and Canale (1991) and adapted in this study to tropical conditions does not consider the sediment and phosphorus resuspension that may occur in the onset of the rainy season, when water inflows reach the empty reservoir. Furthermore, Hart and Harding (2015) argue that bioturbation by fish may impact phosphorus release from bottom sediments, depending on fish stock abundance and lake eutrophic status. Such features, in addition to temperature differences among seasons, may produce variations of the phosphorus setting velocity (v s ) and recycling (v r ) along the year, which was not considered in this study and may represent an additional source of uncertainty in the simulations.…”

“…The sediment reuse meets such demands, presenting the ecological advantage of recycling what is often considered as a waste material and representing a low-cost alternative to the management and disposal of dredged sediment (Bondi et al 2016). Other practices to reduce phosphorus content in lakes have been proposed, as the biomanipulation by fish removal (Hart and Harding 2015), but it is extensively stated that reduction of phosphorus loads to superficial water bodies is more efficient on eutrophication control (Conley et al 2009, Hart andHarding 2015).…”

Seasonal variation of chloro-s-triazines in the Hartbeespoort Dam catchment, South Africa

“…Bioturbation of the hypolimnion due to large quantities of bottom feeding carp and catfish fish in the Hartbeespoort Dam is known as the cause of resuspension of pollutants in the Hartbeespoort Dam (Hart and Harding, 2015). Only DET and DEA were detected in catfish muscle with concentrations of 0.3 and 0.2 ng g −1 respectively.…”

Increasing nutrient influx trends and remediation options at Hartbeespoort Dam, South Africa: a mass-balance approach

“…P removal from fish biomass harvesting was estimated using Eq. 10, assuming a standard of 2.3% P dry mass and dry mass as 22% of wet mass, following Hart and Harding (2015).…”

“…In aquatic ecosystems that are subjected to nutrient enrichment, the reduction or removal (harvesting) of plant and/or animal biomass in order to reduce nutrient stocks and associated nutrient recycling is widely advocated as a strategy to alleviate the problem (Hart and Harding, 2015). The Hartbeespoort Dam Integrated Biological Remediation Programme (HDIBRP), also known as 'Harties Metsi a Me' , which ran from 2006 until 2016, aimed to rehabilitate the dam using this strategy and had a strong food-web restructuring approach.…”