Enclosure Study on Phytoplankton Response to Stocking of Silver Carp (Hypophthalmichthys molitrix) in a Eutrophic Tropical Reservoir in South China

Abstract

A 10-week enclosure experiment was conducted in the dry season (October to December) to assess the impact of fish stocking on water quality and phytoplankton community at three stocking levels of silver carp (Hypophthalmichthys molitrix) (0, 16 and 50 g m -3 ) in a shallow, eutrophic tropical reservoir (Guangdong, South China). The nanophytoplankton (< 20 μm) accounted for 60 ± 5% of total biomass and Scenedesmus quadricauda was the predominant species (size: 16.5 ± 5 μm, relative biomass 27 ± 10%). Phytoplankton biomass was markedly suppressed in high fish stocking level and the grazing effect of silver carp on the nanoplankton was significant from the sixth week to the end of the experiment. The high fish density treatment significantly increased water transparency and somewhat reduced total phosphorus. Indirect effects of silver carp on the phytoplankton community involved excretion of nitrogen that favored small species of Chlorophyta. Colonial Cyanobacteria were reduced in the high fish treatment, but enhanced in the low fish treatment.Our results suggest that the widespread practice of harvesting fish from October to December in reservoirs of South China releases phytoplankton from consumption by filter-feeding fish, and consequently prompts algal blooms. Conversely, harvesting fish just before monsoon, around the time of restocking with small fish, would make more sense since no interruption in grazing pressure would occur. This would help to control excessive phytoplankton biomass just before monsoon.

Remediation of Polluted River Water by Biological, Chemical, Ecological and Engineering Processes

“…However, they require a large area, have low hydraulic load and exhibit intolerance to heavy pollutant loading rate [9], including the observation of seasonal death and plant diseases [68]. Therefore, living plants are recommended to be replaced by aqua mats and artificial seaweed, which can enhance the growth of microorganisms and overcome the above drawbacks [69]. Given the low removal rate of single constructed wetland, Bai et al [9] sequentially constructed floating bed wetland, horizontal subsurface flow constructed wetland and surface flow constructed wetland for the treatment of urban river water.…”

“…For example, the freshwater gastropod was used as a bioindicator to estimate the contamination levels of some heavy metals in river water and sediments [101]. Some aquatic animals, such as clams, snails, silver carp, common carp and other filter-feeding fish, are able to reduce concentrations of nutrients, eutrophication and algae in river water, and thus improve the water quality [69]. However, some studies reported that the capability of silver carp fish to control algal biomass is restricted by the toxic effects of the bio-toxins secreted by algae and by the presence of inorganic or organic contaminants in polluted water, which ultimately hamper the performance of this biological treatment system [83].…”

Effects of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community of a deep, tropical reservoir: an enclosure experiment

“…Unfortunately, high biomasses of silver carp have been generally used in biomanipulation research and few studies have involved low fish biomass (see review by Zhang et al, 2008). below 8 g m )3 ) on the plankton community containing large herbivorous cladocerans, although several studies involving the use of silver carp to control phytoplankton have been conducted (Starling, 1993;Datta & Jana, 1998;Starling et al, 1998;Xiao et al, 2010). For tropical fresh waters, no attempt, to our knowledge, has been made to assess the impact of low biomass of silver carp (e.g.…”

“…For tropical fresh waters, no attempt, to our knowledge, has been made to assess the impact of low biomass of silver carp (e.g. below 8 g m )3 ) on the plankton community containing large herbivorous cladocerans, although several studies involving the use of silver carp to control phytoplankton have been conducted (Starling, 1993;Datta & Jana, 1998;Starling et al, 1998;Xiao et al, 2010).…”

Biomanipulation in mesocosms using silver carp in two Chinese lakes with distinct trophic states

“…Due to their high mass-specific nutrient excretion rate (Vanni, 2002), zooplankton is regarded as an important part of nutrient cycling in the water column. However, zooplanktons are frequently suppressed after the introduction of silver carp (Zhang et al, 2008;Xiao et al, 2010) in mesocosms such as the enclosures in the present study. The absence of large planktivorous zooplankton may reinforce the importance of fish in nutrient cycling.…”