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 llocthonous 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 O 2 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 O 2 L −1 d −1 ), explaining the net heterotrophic nature of the community.