210Po/210Pb dynamics in relation to zooplankton biomass and trophic conditions during an annual cycle in northwestern Mediterranean coastal waters

Abstract

Monthly sampling in northwestern Mediterranean coastal waters was undertaken to better understand the relationship between zooplankton biomass and the cycling of the natural radionuclide 210Po/210Pb pair during a one-year period (October 1995–November 1996). In conjunction with mesozooplankton collections and 210Po/210Pb measurements in seawater, zooplankton and their fecal pellets, the biochemical composition of particulate organic matter (POM) was also examined at three depths (0, 20 and 50 m) as an indicator of trophic conditions. During May 1996, a strong zooplankton “bloom” was observed which was preceded by a prolonged increase in POM (protein + carbohydrates + lipids) starting at the end of March, and further demonstrated by a concomitant increase in the concentration of smaller particles, two features that are typical of mesotrophic waters. Simultaneous measurements of 210Po in sea water and zooplankton showed an inverse trend between these two parameters during the sampling period, with the two lowest 210Po concentrations in the dissolved phase of seawater coincident with the highest radionuclide concentrations in the zooplankton; however, this apparent relationship was not statistically significant over the entire year. Freshly excreted mesozooplankton and salp fecal pellets, which have been strongly implicated in the removal and downward transport of these radionuclides from the upper water column, contained 210Po and 210Pb levels ranging from 175 to 878 and 7.5–486 Bq kg−1 dry weight, respectively. Salp pellets contained 5 and 10 times more 210Po and 210Pb than in fecal pellets produced by mixed zooplankton, a finding most likely related to their different feeding strategies. During the zooplankton biomass peak observed in May, the 210Po concentration in zooplankton was at a minimum; however, in contrast to what has been reported to occur in some open sea oligotrophic waters, over the year no statistically significant inverse relationship was found between zooplankton biomass and 210Po concentration in zooplankton. This observation may have resulted from the general lack of very low biomass concentrations (<1 mg m−3) measured in these coastal waters, biomass levels which commonly occur in open ocean oligotrophic regions.