Regular articleSeasonal and geographic variations in modeled primary production and phytoplankton losses from the mixed layer between warm and cold years on the eastern Bering Sea shelf

The total daily phytoplankton loss from the mixed layer can be estimated as the difference between gross primary production (GPP) and the realized change in phytoplankton carbon biomass. Here a modeling approach is used to estimate the total loss rates for five hydrographic domains on the eastern Bering Sea Shelf during a warm (2000-2006) and a cold (2007-2010) period. Model results indicate that the average daily rate of GPP in the mixed layer for all domains is on average slightly higher than in warm years (950±726 mg C m−2 d−1) than in cold years (859±640 mg C m−2 d−1), but is not significantly different. Similarly, the daily phytoplankton total loss rate from the mixed layer in all domains is on average slightly higher in warm years (961±747 mg C m−2 d−1) than in cold years (888±691 mg C m−2 d−1), but the difference is not significant. That total loss rates show the same warm vs. cold year pattern as GPP, suggests similar seasonal and latitudinal variations and magnitudes of change for both processes. The annual total loss is compared with the sum of individual process losses (e.g., mixing, grazing, sinking, etc.), with the discrepancy being generally larger than ~15% of the total loss both in warm and cold years. The model results also show that annual respiration is generally greater than losses due to zooplankton grazing and sinking both in warm and cold years. Compared among domains, significant differences (t-test, P<0.05) between northern and southern domains (defined as North and South of 60°N) are observed for GPP rate, total daily loss rate and each of the individual loss terms in cold years, while values for southern domains are higher than those of northern domains. In warm years there were no significant differences between domains. Furthermore, these results indicate that total loss rates reflect patterns in GPP rate implying a similar metabolic balance within the ecosystem in both warm and cold years.