Importance of copepod carcasses versus faecal pellets in the upper water column of an oligotrophic area

Abstract

Downward flux of zooplankton faecal pellets and carcasses was studied during and after the spring bloom in an oligotrophic coastal area of the Western Mediterranean using a ‘swimmer-excluding’ sediment trap. Zooplankton detritus retrieved in the trap were comprised of cylindrical faecal pellets (from meso- and macrozooplankton) and copepod carcasses with a respective carbon flux of 0.05–2.69 mg m −2 d −1 and 0.42–4.37 mg m −2 d −1. Carbon and nitrogen flux of carcasses always exceeded that of faecal pellets, except at the beginning of the bloom due to a higher contribution of macrozooplankton faecal material. During the peak of phytoplankton biomass, total faecal flux essentially comprised of copepod faecal pellets (68–86% of the total faecal carbon), whereas before and after this period, macrozooplankton faecal material dominated (88–91% of total faecal carbon flux). Copepod faecal flux was positively correlated with phytoplankton biomass. Estimates of non-predatory biomass mortality rates (from <0.01 to 0.05 d −1) were negatively correlated with chl a with a time lag of 12 days and were lower than predatory mortality values reported in the same area. The paper discusses the relative importance of carcasses versus faecal pellet flux and of non-predatory versus predatory mortality, as well as the potential role of these zooplankton detritus in supporting the production of benthos in oligotrophic areas.