Composition and temporal variation of the near-bottom seston in a Mediterranean coastal area

Trends in seston composition in the near-bottom water layer and the influence of environmental factors were studied in a temperate climate (NW Mediterranean Sea) over an entire annual cycle. Temperature, as well as horizontal and vertical Secchi disk, exhibits seasonal trends near the bottom. River runoff also showed a seasonal trend. Wave height showed a more irregular pattern, but had a tendency to be higher in autumn and spring time. River runoff and wave height explained 57% of the variance in the vertical Secchi disk readings and 44% of the variance in the horizontal Secchi disk readings, suggesting a direct influence on seston concentration in the study area. Chlorophyll a (Chl a), total particulate carbon (TPC) and particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate proteins and carbohydrates in samples collected at a depth of 20 m (0.2e0.5 m above the bottom) every two weeks from May 1997 to August 1998 were analysed. Biochemical analyses were performed on the !100-mm and !10-mm size fractions to evaluate the role of the microplankton and the nano- and picoplankton in seston composition. The !10-mm fraction did not show seasonal trends over the sampling period. Only in spring the Chl a 10e100 mm fraction exhibits a clear microphytoplankton spring bloom. The !10-mm fraction predominated over all the rest of the parameters analysed, accounting for more than 80% of the !100-mm POC and more than 90% of the !100-mm PON. The !10-mm fraction was dominant in the POC over the sampling period, suggesting an important proportion of inorganic material in the 10e100-mm fraction. Protein and carbohydrate carbon made up nearly 50% of the POC over the sampling period, suggesting a rich near-bottom seston composition. The values found were comparable to those recorded in other areas customarily regarded as being richer in terms of both production and biomass. This suggests that in an oligotrophic sea like the Mediterranean, near-bottom seston may be richer than previously thought, and has to be considered to perform annual production balances.