Controls on planktonic metabolism in the Bay of Blanes, northwestern Mediterranean littoral

Long-term and experimental approaches were used to examine the metabolic balance of the planktonic community in the Bay of Blanes (Spanish Mediterranean). Incubation measurements at weekly intervals for 6 yr revealed that community respiration, R, was consistently larger than gross primary production (GPP) by a factor of 2. The plankton community was net heterotrophic for 2/3 of the study period, with a median P/R ratio of 0.65. The biomass of autotrophs comprised, on average, 41% ± 3% of the planktonic biomass, and the total microplankton biomass was about 2.5-fold greater than that of the primary producers. The monthly average GPP and R were positively correlated with day length, and the planktonic respiration and gross production per unit microplankton biomass increased with increasing water temperature. Experimental nutrient additions had a greater effect on GPP than respiration rates; the increase in R along the nutrient gradient was 7.8% (± 0.4%) of the increase in GPP. As a result, net community production increased in parallel with GPP, shifting from net heterotrophic at low GPP to net autotrophic when GPP increased because of nutrient additions. Our results show that the R that would be supported by allocthonous inputs was 3.83 ± 0.67 μmol O ₂ L^-1 d^-1, and the average GPP required to shift the community from net heterotrophic to net autotrophic was about 4 μmol O₂ L^-1 d^-1. This is well above the average GPP recorded in the Bay of Blanes along this study (2.56 ± 0.13 μmol O₂ L^-1 d^-1), explaining the net heterotrophic nature of the community.