Edinburgh Napier University

  Assessment of eutrophication of coastal waters has traditionally relied on bulk indicators of ecosystem state (e.g. nutrients and phytoplankton biomass as chlorophyll) and changes in phytoplankton oristic composition such as the occurrence of nuisance and harmful species. Information on these variables does not allow adequate insight into the effects of anthropogenic nutrient enrichment on ecosystem "health": i.e. the structure and functioning of the biological community. Environmental policies like the Marine Strategy Framework Directive (MSFD) require an ecosystem approach to marine management, suggesting the need for a holistic approach to assessing environmental status. Autotrophic species of microplankton are primary producers and form the base of the pelagic food web. Microheterotrophs are their immediate consumers, and this suggests that changes in microplankton community structure may be a useful indicator of pelagic ecosystem health.
The aim of this study was to develop and test an indicator to detect change in microplankton community structure in the context of eutrophication. The theoretic approach of an existing phytoplankton community index (PCI) was used to develop a microplankton community index (MCI). The theory involved the use of "lifeforms" (functional groups) and system state space theory. The approach was to select lifeforms that provided information on eutrophication, biodiversity and energy flow.
These lifeforms included diatoms, dino agellates, micro-
agellates, and ciliates. Pairs of lifeforms were used as state variables to describe the state of the ecosystem. For each pair of lifeforms, data on abundance or carbon biomass were mapped into state space. The resulting "cloud" of points incorporated the inherent variability of microplankton populations. The index calculated as the difference between "clouds", can be used to determine whether differences occur between diffrent sites (with different degrees of pressure) or at the same site over time (response to pressure at a single site). Three moored instrument sites were selected to develop and test the MCI. High temporal resolution sampling of physical, chemical, and microplankton components was carried out for two years (February 2008 - December 2009) in the western Irish Sea (WIS). For the mooring sites in Liverpool Bay (LBay) in the eastern Irish Sea and the West Gabbard (WGabb) in the southern North Sea data of those components were provided for the same frequency and period.
Microplankton cell abundance and carbon biomass showed that the expected seasonal cycle was coupled to hydrodynamic conditions at each site with the sub-surface light climate considered to be the main factor that controlled the start and duration of the production season at all three sites. At WIS, diatoms dominated the spring bloom and autumn period. Succession from diatoms to dino agellates was associated with increased stratiffication and micro-agellates were abundant but without an obvious seasonal pattern. Diatoms dominated the microplankton throughout the year at LBay and WGabb due to high nutrient concentrations and intermittently stratifying conditions. The influence of nutrient enrichment on microplankton community was investigated at the LBay (≈ 30µM winter DIN) and WGabb (≈ 15µM winter DIN) sites by using five pairs of lifeform state space plots (diatoms/dino agellates, autotrophs/heterotrophs, autotrophs /mixotrophs, mixotrophs/heterotrophs, and small/large sized microplankton). A clear increase in the autotroph biomass at LBay station in the autotrophs/mixotrophs comparison was observed and the MCI value of the small/large sized microplankton comparison suggested a difference between the communities at the two sites with higher biomass of the large sized lifeform at LBay. Comparisons with the heterotrophic lifeform were difficult, because few data points were available.
By including additional microplankton lifeforms the MCI extended the PCI approach and can be used to provide a more complete assessment of change in microplankton community structure. Further development and assessment is required such as what represents the optimum size of datasets for reliable application of the index and the distinction of the nutritional mode in long-term preserved microplankton samples. A key element of the MCI application is the comparison to a reference condition.
According to the MSFD such conditions should be representative of good environmental status (GES). On the basis of current understanding of microplankton ecology (biogeography, seasonal dynamics and succession) the results from this study suggest that the microplankton community at station WIS represents GES and this station is therefore proposed as a reference site for seasonally stratifying temperate shelf seas.