Sinking rates of heterogeneous, temperate phytoplankton populations

Abstract

Throughout the summer of 1978, the sinking rates of phytoplankton within the Controlled Experimental Ecosystems (CEE's) were monitored using a technique based upon measurement of the transit time of radioactively (14C) labeled cells. The experimental framework of FOODWEB 1 offered an unprecedented opportunity to document the sinking rates of heterogeneous phytoplankton of diverse taxonomic composition, growing under a variety of nutrient regimes; the absence of advective exchange in the CEE's provided knowledge of the preconditioning history of the phytoplankton sampled at any given time. Sinking rates of whole phytoplankton assemblages (not size-fractioned) ranged from 0.32 – 1.69 m·day−1; the average rate (± s.d.) observed was 0.64 ± 0.31 m·day−1. The most notable deviations from the mean value occurred when the population size distribution and taxonomic composition shifted due to blooms. The relationship between phytoplankton sinking and ambient nutrient levels was studied by following the rates of a given size fraction (8–53 μm) for ten days following nutrient enrichment of a CEE. Over this time sinking rates ranged from 1.08– 1.53 m·day−1; decreased rates occurred after nutrification, yet over the course of the entire trial sinking rates were not significantly (p >0.05) correlated to the ambient levels of any single nutrient species. The sinking rate implications of spore formation were also studied, and showed that sinking rates of Chaetoceros constrictus and C. socialis spores (2.75 ± 0.61 m·day−1) were ca 5-fold greater than rates measured when the vegetative stages of these species dominated the population, reflecting the influence of physiological mechanisms in controlling phytoplankton buoyancy. An example of the potential influence of colony formation upon buoyancy was noted in observations of C. socialis which occasionally was found to exist in large spherical configurations made of coiled cell chains and having low (≤0.40 m·day−1) sinking rates. A hydrodynamic rationale is presented to show how such a colony together with enveloped water may behave as a unit particle having lower excess density, and therefore low observed sinking rate, despite its conspicuously large size. Overall, sinking rates were not significantly correlated with any of the measured nutrient or photic parameters. There were, however, trials and comparisons which showed evidence for: (1) higher sinking rates in populations dominated by large cells, (2) decreased sinking rates after nutrient enrichment, and (3) buoyancy response to light levels. It is suggested that correlation of sinking rates with synoptic environmental measurements at any given time is not explicit because the associations may invoke lag times of physiological response. The interpretation made from these findings is that the preconditioning history of the population, rather than the prevailing conditions at the time of a given measurement, is most closely associated with population buoyancy modifications.