Microbial size spectra from diverse marine ecosystems

Abstract

Characteristics of microbial size spectra (bacteria and phytoplankton) were examined in relation to changes in ecosystem productivity and environmental perturbations. Samples were obtained from productive coastal waters in Massachusetts and Cape Cod Bays, oligotrophic waters in the Sargasso Sea and high nutrient, low chlorophyll waters in the equatorial Pacific. In general, a relative predominance of larger bacteria and phytoplankton cells was observed in early spring, where low temperatures resulted in wellmixed waters and high nutrient concentrations. Seasonal succession was accompanied by a shift in the size spectrum to smaller cells, coinciding with rising temperatures, stratification of the water column and diminishing nutrient concentrations. In stratified waters, larger mean bacteria and phytoplankton sizes were observed in surface and deep waters, whereas smaller sizes were observed around the chlorophyll maximum. Bacteria and phytoplankton growth were well correlated with mean bacteria sizes varying positively with mean phytoplankton sizes. Data pooled from all locations showed that the size spectral characteristics most sensitive to environmental change were the mean cell size, bacteria intercept and phytoplankton slope of the normalized concentration size spectrum. Increases in ecosystem productivity, chlorophyll, particulates and nutrients were generally accompanied by shifts in the size spectra to larger bacteria and phytoplankton.