Partial recovery of a eutrophic reservoir through managed phosphorus limitation and unmanaged macrophyte growth

An 18-day mesocosm study was performed in central Norway to assess the effect of enhanced nutrient input to the marine plankton community. This paper reports the responses of micro- and mesozooplankton to increased food supply due to elevated nutrient input. Seven mesocosms (M1–M7) were added variable doses of N, ranging from 0 to 2.19 m N l^–1 d^–1. Phosphorus and silicate was added in Redfield ratios. The ciliate community responded rapidly to the treatment, and reached maximum biomass of 88 g C l^–1 within a week in the most fertilised mesocosm (M7). Tontonia sp. and a small Strombidium sp. dominated biomass and numbers, respectively. Ciliate biomass declined rapidly after the peak, returning to initial values by the end of the experiment (Day 18). Mesozooplankton biomass increased from the second week, due to recruitment of Acartia spp., Centropages spp. and Oithona sp. Numbers of Temora longicornis and Pseudocalanus sp. remained low. Highest biomass of mesozooplankton (116 g C l^–1) was recorded in M6 by Day 18. Egg production rates for Acartia spp. peaked in M3 at Day 11, while calculated mortality rates for juvenile copepods was highest in M1 and M7. Estimated community net growth rates were highest in the most fertilised mesocosms for both copepods and ciliates. It is concluded that enhanced nutrient input affected both biomass and the relative species composition of the zooplankton community.