Influence of timing of sea ice retreat on phytoplankton sizeduring marginal ice zone bloom period on the Chukchi and Bering shelves

[摘要] The size structure and biomass of a phytoplankton community during the springbloom period can affect the energy use of higher-trophic-level organismsthrough the predator–prey body size relationships. The timing of the sea iceretreat (TSR) also plays a crucial role in the seasonally ice-covered marineecosystem, because it is tightly coupled with the timing of the spring bloom.Thus, it is important to monitor the temporal and spatial distributions of aphytoplankton community size structure. Prior to this study, an ocean colouralgorithm was developed to derive phytoplankton size index F_L,which is defined as the ratio of chlorophyll a (chl a) derived from cellslarger than 5 µm to the total chl a, using satellite remotesensing for the Chukchi and Bering shelves. Using this method, we analysedthe pixel-by-pixel relationships between FL during the marginal icezone (MIZ) bloom period and TSR over the period of 1998–2013. The influencesof the TSR on the sea surface temperature (SST) and changes in ocean heatcontent (ΔOHC) during the MIZ bloom period were also investigated. Asignificant negative relationship between F_L and the TSR was widelyfound in the shelf region during the MIZ bloom season. However, wefound a significant positive (negative) relationship between the SST (ΔOHC) and TSR. Specifically, an earlier sea ice retreat was associated withthe dominance of larger phytoplankton during a colder and weakly stratifiedMIZ bloom season, suggesting that the duration of the nitrate supply, whichis important for the growth of large-sized phytoplankton in this region (i.e.diatoms), can change according to the TSR. In addition, under-icephytoplankton blooms are likely to occur in years with late ice retreat,because sufficient light for phytoplankton growth can pass through the iceand penetrate into the water columns as a result of an increase in solarradiation toward the summer solstice. Moreover, we found that both the lengthof the ice-free season and the annual median of F_L positivelycorrelated with the annual net primary production (APP). Thus, both thephytoplankton community composition and growing season are important for theAPP in the study area. Our findings showed a quantitative relationshipbetween the interannual variability of F_L, the TSR, and the APP,which suggested that satellite remote sensing of the phytoplankton communitysize structure is suitable to document the impact of a recent rapid sea iceloss on the ecosystem of the study region.

[学科分类] 地球化学与岩石