MEPS 499:19-34 (2014)  -  DOI: https://doi.org/10.3354/meps10633

ABSTRACT: Seasonal respiratory oxygen consumption was measured by bottle incubation of bottom water from 5 stations in transects across the Baltic Sea–North Sea transition zone. Respiration was measured at 3 temperatures (in situ temperature and in a 5°C colder and 5°C warmer incubation), to determine temperature sensitivity. The seasonal range in oxygen consumption was 9 to 90 mg O₂ m^-3 d^-1 corresponding to mineralization rates between 2.8 and 28 mg organic C m^-3 d^-1. The total below-halocline mineralization was estimated to be 53 g organic C m^-2 yr^-1 in the area; and 38 g C m^-2 yr^-1 when the estimate was adjusted to the mean annual temperature. Temperature sensitivity, expressed in terms of a Q₁₀ value, was 3.0 ± 1.1 averaged over 23 experiments. The specific decay rate of organic C ranged between 0.0027 and 0.094 d^-1. To determine the seasonal lability of organic matter, the specific decay rates were normalized to a reference temperature of 10°C using the observed Q₁₀ values, and these estimates showed a range of decay rates between 0.0040 d^-1 (January) and 0.049 d^-1 (August). The C/N ratio of particulate organic matter (POM) ranged between 7 and 22. The highest decay rates were associated with low (<10) C/N ratios; the C/N ratios increased during the incubations. Measurements of the POM pool showed no decline during the incubations. The initial pool of POM in the bottles could not account for the observed oxygen consumption—which on average corresponded to 183% of the POM pool, indicating that dissolved organic matter (DOM) is the most important carbon source for bottom water respiration in this area.

KEY WORDS: Respiration · Kattegat · Carbon budget · Incubations · Bottom water · Q₁₀