The Ecology of Sediment Bacteria in Lakes and Comparisons with Other Aquatic Ecosystems

We have analyzed our own data set as well as a number of previously published ones to examine factors related to the numbers and biomass of sediment bacteria in a variety of ecosystems. We have chosen to express sediment variables either on an areal or per unit sediment volume basis to avoid bias due to the more common standardization to sediment dry mass (also known as gram dry weight). Cross—system comparisons show that the amount of organic matter present and the sediment water content are important to sediment bacterial abundance. The C:N ratio of the sediment organic matter, possibly reflecting organic matter quality, was found to explain variation in bacterial numbers in addition to that explained by organic content. Over a broad range, sediment water content is related to sediment bacterial numbers in a unimodal fashion indicating dilution of sediments by water at high (>80%) water content sites and dilution by inorganic particles at low (<50%) water content sites. In lakes, the most important factors explaining sediment bacterial biomass were sediment water content and hydraulic flushing rate, a finding supported by analysis of an independent data set of English lakes. Planktonic chlorophyll a and planktonic bacterial numbers (both reflecting autochthonous organic matter levels) were not significantly related to sediment bacterial biomass. We found that sediments overlain by epilimnetic water had significantly higher bacterial biomass than those overlain by hypolimnetic water. A strong positive relationship between macrobenthos and sediment bacterial biomass was found for lakes in Ontario and Quebec. Calculations of annual bacterial production and P:B ratios necessary to sustain the macrobenthic population, based on the observed bacterial and macrobenthic biomass and temperature at our sites, indicate that the bacterial biomass present is more than sufficient to maintain the macrobenthos. In addition, based on the observed ratios of bacterial to detrital carbon at our sites and on a range of published assimilation efficiencies for macrobenthos grazing on these carbon sources, we calculated that the mean percentage of carbon of bacterial origin assimilatable by macrobenthos at our sites is 53% with a maximum of 79% and a minimum of 12%. Our results suggest a tighter trophic linkage between sediment bacteria and macrobenthos in our lakes than in most marine habitats, as indicated by published studies. From these analyses, the general picture emerges of a decoupling between pelagic and benthic bacterial processes in lakes, the pelagic ones being driven mainly by autochthonous production and the benthic ones by allochthonous inputs.