Abstract
Increased recruitment of small-sized fish following biomanipulation by reducing the biomass of plankti-benthivorous fish, not least in (sub)tropical lakes, may deteriorate water quality and thereby potentially hamper the recovery of submerged macrophytes. Filter-feeding bivalves remove suspended particles from the water and may, thereby, somewhat or fully counteract this negative effect of the increasing abundance of small-sized fish. So far, only few studies have investigated the interactive effects of fish and bivalves on water clarity and macrophyte growth. We conducted a 2 × 2 factorial designed outdoor mesocosm experiment with two densities of small crucian carp Carassius carassius (low 10 g m−2 and high 40 g m−2) and two densities of bivalves Corbicula fluminea (low 204 g m−2 and high 816 g m−2). We found significant interactive effect of fish and bivalves on the growth of the macrophyte Vallisneria natans. In the low density bivalve regime, the relative growth rates, root mass, root:shoot ratio and number of tubers were 30.3%, 30.8%, 21.6% and 27.8% lower in the high than in the low density fish treatments, while the decrease was less pronounced in the high density bivalve regime: 1.2%, 8.7%, 2.1% and 13.3%, respectively. Thus, bivalves reduced the negative effects of fish, not least when bivalve density was high. The weaker effects of small fish on plants in the high- than in the low-density C. fluminea regime can be attributed to lower total suspended solids (TSS) and Chl a in the first week of the experiment. Better light conditions further stimulated the growth of benthic algae, potentially increasing the removal of nutrients from the water and reducing fish-driven resuspension of the sediment. In addition, high densities of C. fluminea also enriched the sediment total nitrogen (TN) and total phosphorus (TP) content, favouring plant growth as indicated by an increase in leaf tissue TN and TP contents. Our results demonstrate that filter-feeding bivalves can alleviate harmful effects of small fish by prolonging a clear-water state that facilitates submerged macrophyte growth. Addition of the bivalve C. fluminea can be a promising tool for the restoration of submerged macrophytes in shallow eutrophic lakes, in particular lakes containing small, rapidly reproducing fish that due to their small sizes are not capable of controlling the bivalves.
Highlights
Biomanipulation, targeted at obtaining a substantial reduction of planktivorous and benthivorous fish, has been widely used to improve water quality and promote re-establishment of submerged macrophytes [1,2,3]
In the low-density bivalve regime, high fish density led to rapid increases in total nitrogen (TN) and total phosphorus (TP) concentrations during the first week, with mean concentrations of 27.7% and 25.4%, respectively, which was significantly higher than at low fish density (p < 0.01)
Our results support the hypothesis that high abundance of small-sized crucian carp decreases the growth of the submerged macrophyte V. natans, while filtration by the bivalve C. fluminea partly alleviates the negative effects of the fish on plant growth
Summary
Biomanipulation, targeted at obtaining a substantial reduction of planktivorous and benthivorous fish, has been widely used to improve water quality and promote re-establishment of submerged macrophytes [1,2,3]. Submerged macrophytes are considered to play a fundamental structuring role in shallow lakes [4,5,6], and when abundant help to sustain a long-term clear-water state after restoration [7,8], as they provide a refuge for zooplankton against fish predation, with a higher grazing pressure on phytoplankton; they further protect young piscivores from predation, reduce sediment resuspension and inhibit phytoplankton growth via nutrient and light competition and allelopathy. Successful biomanipulation achieved in some temperate lakes (e.g., in the Netherlands and Denmark), has been attributed to increased development and cover of submerged macrophytes [9,10,11]. Fish manipulation may not have the same positive effects on macrophyte re-establishment in warm as in temperate lakes [12,13]. How to control the abundance is of key relevance restoration of warm lakes, but so far not well studied
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