Arctic mid-winter phytoplankton growth revealed by autonomous profilers

It is widely believed that during winter and spring, Arctic marine phytoplankton cannot grow until sea ice and snow cover start melting and transmit sufficient irradiance, but there is little observational evidence for that paradigm. To explore the life of phytoplankton during and after the polar night, we used robotic ice-avoiding profiling floats to measure ocean optics and phytoplankton characteristics continuously through two annual cycles in Baffin Bay, an Arctic sea that is covered by ice for 7 months a year. We demonstrate that net phytoplankton growth occurred even under 100% ice cover as early as February and that it resulted at least partly from photosynthesis. This highlights the adaptation of Arctic phytoplankton to extreme low-light conditions, which may be key to their survival before seeding the spring bloom.

Mesozooplankton grazing minimally impacts phytoplankton abundance during spring in the western North Atlantic

The impacts of grazing by meso- and microzooplankton on phytoplankton primary production (PP) was investigated in the surface layer of the western North Atlantic during spring. Shipboard experiments were performed on a latitudinal transect at three stations that differed in mixed layer depth, temperature, and mesozooplankton taxonomic composition. The mesozooplankton community was numerically dominated by Calanus finmarchicus at the northern and central station, with Calanus hyperboreus also present at the northern station. The southern station was >10 °C warmer than the other stations and had the most diverse mesozooplankton assemblage, dominated by small copepods including Paracalanus spp. Microzooplankton grazing was detected only at the northern station, where it removed 97% of PP. Estimated clearance rates by C. hyperboreus and C. finmarchicus suggested that at in-situ abundance these mesozooplankton were not likely to have a major impact on phytoplankton abundance, unless locally aggregated. Although mesozooplankton grazing impact on total phytoplankton was minimal, these grazers completely removed the numerically scarce > 10 µm particles, altering the particle-size spectrum. At the southern station, grazing by the whole mesozooplankton assemblage resulted in a removal of 14% of PP, and its effect on net phytoplankton growth rate was similar irrespective of ambient light. In contrast, reduction in light availability had an approximately 3-fold greater impact on net phytoplankton growth rate than mesozooplankton grazing pressure. The low mesozooplankton grazing impact across stations suggests limited mesozooplankton-mediated vertical export of phytoplankton production. The constraints provided here on trophic transfer, as well as quantitative estimates of the relative contribution of light and grazer controls of PP and of grazer-induced shifts in particle size spectra, illuminate food web dynamics and aid in parameterizing modeling-frameworks assessing global elemental fluxes and carbon export.

Estimating the carbon biomass of marine net phytoplankton from abundance based on samples from China seas

The relationship between carbon biomass and cell abundance in net phytoplankton was determined to improve standing stock research in marine ecology. Based on samples from six cruises in the Yellow Sea and the East China Sea, significant regression equations for all net phytoplankton cells, diatoms, dinoflagellates and each dominant genus were obtained. The relationships could be described by the equation log10y=k×log10x+b, where x represents cell abundance based on cell counts (cells m–3), y represents carbon biomass (μgCm–3), and k and b are constants. The values of k and b were 0.48 and 0.49 respectively for total net phytoplankton, 0.75 and –1.46 respectively for diatoms in summer, 0.54 and –0.11 respectively for diatoms in spring and autumn, and 0.92 and –0.90 respectively for dinoflagellates. Regression equations for Chaetoceros, Coscinodiscus, Pseudo-nitzschia, Skeletonema, Ceratium, Protoperidinium and Pyrophacus were also obtained. We suggest using these carbon biomass:cell abundance relationships established for net phytoplankton to assess phytoplankton standing stocks and for reanalysing historical data.

A Study on the Distribution of Net-Phytoplankton and Nutrients [N, P] of Xiangshan Bay in Summer

The distributions of net-phytoplankton and nutrients were studied in Xiangshan Bay in summer, 2012. The results showed that in summer, the higher values of net-phytoplankton cell abundance were distributed in the areas of Tie port, Huangdun port and Xihu Port. Whereas, the lower values were mainly found in the stations located in the outer area of Xiangshan Bay and the area around Liuheng Island. The content of nitrate in the middle of the bay was higher than that both in the bottom and the entrance of the bay. The concentration of nitrate in the bottom water was a little higher than that in the surface water. The distribution trends of the concentration of nitrite, ammonium and phosphate were almost the same. The higher values were discovered in the sea area of Tie port and Huangdun port and the lower were mainly distributed in the area of the outer Xiangshan Bay and the sea area around Liuheng Island. The distribution trends were almost the same no matter in the surface water or in the bottom water. From the bottom to the middle to the entrance of the bay, the values reduced in turn. In summer, in the water of Xiangshan Bay, net-phytoplankton cell abundance and nutrient (N, P) content didn’t have significant correlation. The concentration of the nutrients (Nitrate, Nitrite, Ammonium and Phosphate) was far more than the minimum threshold of the growth of phytoplankton.