Synergistic negative effects of small-sized benthivorous fish and nitrogen loading on the growth of submerged macrophytes - Relevance for shallow lake restoration.

Rapid recruitment of small fish after biomanipulation in warm lakes may delay the reestablishment of submerged macrophytes, not least at high nutrient concentrations. Success has recently been obtained in controlling phosphorus (P) loading to many lakes, but nitrogen (N) inputs often remain high. To determine the interactive effects of N loading and the abundance of small-sized fish on the growth of the submerged macrophyte Vallisneria natans, we conducted an outdoor mesocosm experiment with a factorial design on the north shore of Lake Taihu, China. The experiment involved two densities of small crucian carp - low (10gm-2) and high (40gm-2) - crossed with two levels of N loading - present-day external nutrient loading (P: 5μgL-1day-1, N: 130μgL-1day-1) and P: 5μgL-1day-1 with a three times higher N loading (N: 390μgL-1day-1). The results showed that nitrogen-fish interactions significantly hindered the growth of V. natans, particularly at the high N loading. At low N loading, high densities of fish decreased the relative growth rate, mean leaf length, leaf mass and root mass of V. natans by 16%, 5%, 8%, and 23%, respectively, compared with these measures at low fish densities. The effect of fish was even stronger when N loading was high, with decreases of 232%, 32%, 57%, and 47% for the respective plant growth measures. The stronger effect at high N loading was attributed to higher turbidity due to enhanced phytoplankton biomass and to increased consumption or damage of plants by the fish in response to the more nutrient-enriched plant tissue. Our results indicate that high abundance of small crucian carp in warm lakes may reduce the resilience of submerged macrophytes to external N loading, thereby lowering the chances of successful restoration by biomanipulation.