Abstract
Phytoplankton blooms are usually dominated by chain-forming diatom species that can alter food pathways from primary producers to predators by reducing the interactions between intermediate trophic levels. The food-web modifications are determined by the length of the chains; however, the estimation is biased because traditional sampling strategies damage the chains and, therefore, change the phytoplankton size structure. Sedimentological studies around oceanic fronts have shown high concentrations of giant diatom mats (>1 cm in length), suggesting that the size of diatom chains is underestimated in the pelagic realm. Here, we investigate the variability in size and abundance of phytoplankton chains at the Ushant tidal front (NW France) using the Video Fluorescence Analyzer (VFA), a novel and non-invasive system. CTD and Scanfish profiling characterized a strong temperature and chlorophyll front, separating mixed coastal waters from the oceanic-stratified domain. In order to elucidate spring-neap variations in the front, vertical microstructure profiler was used to estimate the turbulence and vertical nitrate flux. Key findings were: (1) the VFA system recorded large diatom chains up to 10.7 mm in length; (2) chains were mainly distributed in the frontal region, with maximum values above the pycnocline in coincidence with the maximum chlorophyll; (3) the diapycnal fluxes of nitrate enabled the maintenance of the bloom in the frontal area throughout the spring-neap tidal cycle; (4) from spring to neap tide the chains length was significantly reduced; (5) during neap tide, the less intense vertical diffusion of nutrients, as well as the lower turbulence around the chains, intensified nutrient-depleted conditions and, thus, very large chains became disadvantageous. To explain this pattern, we suggest that size plasticity is an important ecological trait driving phytoplankton species competition. Although this plasticity behavior is well known from experiments in the laboratory, it has never been reported from observations in the field.
Highlights
Food web dynamics and the carbon cycle are largely controlled by the size structure of the phytoplankton community [1,2,3]
Environmental Conditions Based on their hydrological properties, three significantly different groups of stations (P,0.001) were distinguished using similarity profile routine (SIMPROF), at a Euclidean distance similarity of 4.32 (Figs. 1b, c)
The clusters resulting from the SIMPROF test were superimposed on the multidimensional scaling (MDS) plot, indicating proper separation between the three groups (Figs. 1b, c): (M) contained stations, located in the wellmixed area (2–6 and 18–23); (F) grouped stations located in the front (7–11 and 24–27); and (S) was made up of stations sampled in the offshore strong stratified region (12–16 and 28–34)
Summary
Food web dynamics and the carbon cycle are largely controlled by the size structure of the phytoplankton community [1,2,3]. Chain-forming species of diatom in particular display broad size plasticity, occurring solitarily or in colonies of extensive lengths [5,6,7]. They have the ability to grow rapidly under elevated nitrate and silicate concentrations, forming blooms [8,9]. During these blooms, the dominance of long chains alters the food pathways from primary producers to predators, reducing the interactions between intermediate trophic levels. Density gradients and the presence of spines or cell projections can slow gravitational settling [10,11,12], their fate is to passively sink out of the euphotic zone, whilst exporting to the bottom fixed CO2 [2]
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