Detritus and Juvenile Salmon Production in the Nanaimo Estuary: III. Importance of Detrital Carbon to the Estuarine Ecosystem

Abstract

Sources of autotrophic and allochthonous organic carbon available to the Nanaimo Estuary delta, British Columbia, were studied from 1974 to 1978. Annually, benthic microalgae produce 4–55 g C∙m−2, phytoplankton ~7.5 g C∙m−2, and macroalgae 0.9–7.5 g C∙m−2. Zostera marina (26.8 g C∙m−2∙yr−1) and Carex (~564 g C∙m−2∙yr−1) are productive but enter the food web as detritus. Allochthonous sources are most important with organic matter from the river, especially dissolved organic carbon (~2000 g C∙m−2∙yr−1) and fine particulates (56 g C∙m−2∙yr−1), contributing greatest amounts. The standing crop of organic detritus in the top 5 cm of sediments averages from 58 to 233 g C∙m−2, depending upon the station. The timing of organic inputs are important, however. Seventy to 93% of total annual river inputs occur during autumn freshets, Zostera enters the food web during winter, Carex may contribute in early spring, and algae are productive over summer months. Activity and biomass of microbes are high most of the year in surface sediments but in the water column microbes are relatively active only during warmer months. Surface sediment ATP concentrations range seasonally from 3 to 36 μg∙g sediment−1; and concentrations in the water column range from < 0.2 to 1.5 μg∙L−1. Microbial activity, measured with 14C-glucose, ranges seasonally from 4 to 20% uptake per hour for surface sediments and from < 1 to nearly 40%∙h−1 in the water column. These results are compared with those from other studies and coupled to concurrent studies of meiofauna and salmon ecology to show a link between detritus, microbes, harpacticoid copepods, and the food, growth, and production of juvenile chum salmon (Oncorhynchus keta). Key words: detritus, Nanaimo Estuary, primary production, carbon, microbes, salmon, algae, ATP, chlorophyll, Zostera