Spatial patterns of phytoplankton composition and upper-ocean biogeochemistry do not follow carbonate chemistry gradients in north-west European Shelf seas

A key difficulty in ocean acidification research is to predict its impact after physiological, phenotypic, and genotypic adaptation has had time to take place. Observational datasets can be a useful tool in addressing this issue. During a cruise in June-July 2011, measurements of upperocean biogeochemical variables, climatically active gases and plankton community composition were collected from northwestern European seas. We used various multivariate statistical techniques to assess the relative influences of carbonate chemistry and other environmental factors on these response variables. We found that the spatial patterns in plankton communities were driven more by nutrient availability and physical variables than by carbonate chemistry. The best subset of variables able to account for phytoplankton community structure was the euphotic zone depth, silicic acid availability, mixed layer average irradiance, and nitrate concentration (59% of variance explained). The spatial variations in phytoplankton and coccolithophores species composition were both found to be more strongly associated with nutrients and physical variables than carbonate chemistry, with the latter only explaining 14 and 9% of the variance, respectively. The plankton community composition and contribution of calcifying organisms was not observed to change under lower calcite saturation state (Ω) conditions, although no regions of undersaturation (Ω<1) were encountered during the cruise. Carbonate chemistry played a more prominent, but still secondary, role in determining dinoflagellate and diatom assemblage composition (20 and 13% of total variance explained, respectively). Nutrient and physical variables also explained more of the spatial variations of most climatically active gases and selected biogeochemical response variables, although some also appeared to be influenced by carbonate chemistry. This observational study has demonstrated that ocean acidification research needs to be set in context with other environmental forcing variables to fully appreciate the primary, or indeed secondary, role that increasing fugacity of carbon dioxide has on biological communities and associated biogeochemical rates.