Short-time-scale variability of near-bottom seston composition during spring in a warm temperate sea

Abstract

In this study, environmental (temperature, wave height, river run-off, and currents) and biological (Chlorophyll a (Chl a), total particulate carbon (TPC), particulate organic carbon (POC), particulate organic nitrogen (PON), proteins, and carbohydrates) features were analysed in Mediterranean coastal waters using a short-time-scale approach. In order to explain shifts in the biomass variability within the spring (a period of intense production in the Mediterranean), samplings were made in March and June (considered extremes in the environmental features of this season). The near-bottom water layers (0.2–0.5 m above the bottom) were sampled each 4 h (March) and 6 h (June) to understand the short-time-scale variation of the plankton–benthos relationship in this time of the year. While Chl a and Carbohydrate concentrations were higher in March, POC and Protein concentrations were higher in June. Higher Variation Indices were found in June with respect to March in almost all the analysed parameters (environmental and biochemical). This point is reflected in the biochemical parameters, which were more related to each other in June than in March. Horizontal Secchi Disk (HSD) readings (water transparency, considered a good descriptor of particle abundance) were linked to environmental factors (i.e. wave height and river run-off). The activity of five of the six passive suspension feeders observed in this time of the year was significant correlated with such HSD readings only in June. Our findings suggest that the variable patterns of environmental factors in spring (especially in June), increase turbulence near the bottom, and alter seston composition and perhaps biological production, which gives more available food for benthic communities in June than in March. We hypothesize that higher biomass and variability in the concentration of biochemical parameters in the near-bottom water layers, together with frequent pulses in the environmental factors in spring may be one of the key factors in explaining the high production levels of benthic communities at this time of the year.