Ingrid Gismervik

Micro- and mesozooplankton response to enhanced nutrient input - a mesocosm study

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 μm 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 μg 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 μg 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.

Comparative analysis of food webs based on flow networks: effects of nutrient supply on structure and function of coastal plankton communities

The objective of COMWEB was to develop efficient analytical, numerical and experimental methods for assessing and predicting the effects of nutrient (N, P, Si) supply on the stability and persistence of pelagic food web structure and function in coastal waters. The experimental comparative work included a geographic gradient covering Baltic, Mediterranean, and NE Atlantic waters and a NE Atlantic gradient in state of eutrophication. COMWEB has been an experimental approach to coastal eutrophication, studying effects of enhanced nutrient supply on components and flows of the entire lower pelagic food web. Flow network representations of pelagic food webs has been a framework of data reduction and flows were established by sophisticated inverse modelling. Fundamental information on physiological properties of functional key species in the pelagic food web was used to constrain flow estimations. A main conclusion derived from the flow networks was that very little energy and materials were transferred from the microbial food web to the main food chain. The lower food web could therefore be described as two parallel food chains with relatively limited interaction between heterotrophic groups. Short-term effects of nutrient perturbations were examined in mesocosms along the geographic gradient. The response was comparable in all systems, with a stronger effect on the activity and biomass of autotrophic groups than those of heterotrophic ones. Mediterranean waters showed much lower autotrophic biomass response than Baltic and NE Atlantic waters, which responded almost equally. The response of primary production was, however, more comparable. High phytoplankton lysis rate explained this low accumulation of biomass in Mediterranean waters. The study of Atlantic coastal waters of different eutrophic states revealed that the ecological response was higher in the closed nutrient perturbed mesocosms than in open systems exposed for >4 summer months (summer/autumn season). The Atlantic lagoon evolved gradually from the natural oligotrophic situation towards the more eutrophicated North Sea during fertilisation. The responses observed on seasonal and long-term scale (>10 years) may therefore be equal. The differences between short-term (weeks) and intermediate-term (seasonal) responses is most likely a result of the different time scales of perturbation and observation and the variable exchange rates with surrounding waters (water dilution rate). The analysis of pelagic flow networks provided a framework of diagnostic criteria for state and quality assessment of coastal waters. The nutrient loading rates related better to estimates of biotic fluxes than to concentrations of biotic compartments and total nutrients. On the contrary, the concentration of biotic compartments, or the biomasses, related better to total nutrient concentrations. Primary production, mesozooplankton grazing and growth, fraction of primary production consumed by grazers, bacterial production relative to primary production, cycling indices, and path lengths were all well related to nutrient loading rate. Autotrophic biomass, ratio of autotrophic to heterotrophic biomass, and fraction of pico-cyanobacteria of total autotrophic biomass were all related to total nutrients. Some of these variables, which responded equally in all systems, have the potential of becoming unified response functions in a management model for European coastal waters. COMWEB has provided further insight into the mechanisms behind coastal eutrophication. A main achievement is the conceptual framework for unified response functions, important components of management models for nutrient emission to coastal waters.