Seasonal and spatial variations in pelagic community respiration on the southeastern U.S. continental shelf

Pelagic microbial community respiratory rates on the southeastern continental shelf show strong seasonal changes, with summer maxima and winter minima. In nearshore waters <20 m, community respiratory rates of 0.1–3 μM O2 l−1 h−1 are similar to recent estimates of primary production. In outer shelf water >40 m, strongly influenced by the Gulf Stream, community respiration is 0.1–2.7 μM O2 l−1 h−1, exceeding primary production in summer. The changes in microbial community respiration are partially explained by changes in bacterial numbers. A related potential cause is the inverse relationship between limiting substrate concentration and temperature, affecting rates of growth and respiration, that has been found to exist for aerobic heterotrophy bacterial isolates in the laboratory. These processes result in shifts between net autotrophy in winter and net heterotrophy in summer for the community as a whole, with little excess organic production remaining for export to the oceanʹs interior.