Abstract
Phytoplankton underpin the marine food web in shelf seas, with some species having properties that are harmful to human health and coastal aquaculture. Pressures such as climate change and anthropogenic nutrient input are hypothesized to influence phytoplankton community composition and distribution. Yet the primary environmental drivers in shelf seas are poorly understood. To begin to address this in North Western European waters, the phytoplankton community composition was assessed in light of measured physical and chemical drivers during the “Ellett Line” cruise of autumn 2001 across the Scottish Continental shelf and into adjacent open Atlantic waters. Spatial variability existed in both phytoplankton and environmental conditions, with clear differences not only between on and off shelf stations but also between different on shelf locations. Temperature/salinity plots demonstrated different water masses existed in the region. In turn, principal component analysis (PCA), of the measured environmental conditions (temperature, salinity, water density and inorganic nutrient concentrations) clearly discriminated between shelf and oceanic stations on the basis of DIN∶DSi ratio that was correlated with both salinity and temperature. Discrimination between shelf stations was also related to this ratio, but also the concentration of DIN and DSi. The phytoplankton community was diatom dominated, with multidimensional scaling (MDS) demonstrating spatial variability in its composition. Redundancy analysis (RDA) was used to investigate the link between environment and the phytoplankton community. This demonstrated a significant relationship between community composition and water mass as indexed by salinity (whole community), and both salinity and DIN∶DSi (diatoms alone). Diatoms of the Pseudo-nitzschia seriata group occurred at densities potentially harmful to shellfish aquaculture, with the potential for toxicity being elevated by the likelihood of DSi limitation of growth at most stations and depths.
Highlights
The shelf seas occupy only approximately 10uC of the world ocean, neritic phytoplankton contribute about a quarter of global primary production [1] that underpins marine food webs and regional fisheries, and represent a significant contribution to global carbon cycling [2]
Due to the numerical dominance of diatoms, detailed taxonomy within our study focused on this group, dinoflagellates were enumerated in two size classes (, 20 mm and . 20 mm) with relatively large distinctive dinoflagellates species belonging to the genera Ceratium, Dinophysis or Prorocentrum being identified to species
Phytoplankton biomass and functional groups Phytoplankton biomass concentrations, as assessed by chl a, were typical of the region in autumn [32], with our peak chlorophyll concentration of 2.3 mg chl a m23 being approximately half the value reported by Savidge & Lennon [35] during their August study of the same transect
Summary
The shelf seas occupy only approximately 10uC of the world ocean, neritic phytoplankton contribute about a quarter of global primary production [1] that underpins marine food webs and regional fisheries, and represent a significant contribution to global carbon cycling [2]. Coastal waters in Northern Europe are under a range of pressures including climate driven temperature change, nutrient enrichment and pollution [3] These and other environmental factors have been suggested to influence the temporal and/or spatial distribution of phytoplankton functional groups and even of particular species. Experiencing relatively little anthropogenic influence, the Scottish continental shelf is a potentially important reference region for policy makers implementing regulations within the EU Marine Strategy Framework and Water Framework directives. Study of this relatively un-impacted area will provide a baseline against which to address future ecosystem change [4]
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