No abstract

Using the seawater dilution technique, we measured phytoplankton growth and microzooplankton grazing rates within and outside of the 1999 Bering Sea coccolithophorid bloom. We found that reduced microzooplankton grazing mortality is a key component in the formation and temporal persistence of the Emiliania huxleyi bloom that continues to proliferate in the southeast Bering Sea. Total chlorophyll a (Chl a) at the study sites ranged from 0.40 to 4.45 mg C l À1 . Highest phytoplankton biomass was found within the bloom, which was a mixed assemblage of diatoms and E. huxleyi. Here, 75% of the Chl a came from cells >10 mm and was attributed primarily to the high abundance of the diatom Nitzschia spp. Nutrient-enhanced total phytoplankton growth rates averaged 0.53 d À1 across all experimental stations. Average growth rates for >10 mm and o10 mm cells were nearly equal, while microzooplankton grazing varied among stations and size fractions. Grazing on phytoplankton cells >10 mm ranged from 0.19 to 1.14 d À1 . Grazing on cells o10 mm ranged from 0.02 to 1.07 d À1 , and was significantly higher at non-bloom (avg. 0.71 d À1 ) than at bloom (avg. 0.14 d À1 ) stations. Averaged across all stations, grazing by microzooplankton accounted for 110% and 81% of phytoplankton growth for >10 and o10 mm cells, respectively. These findings contradict the paradigm that microzooplankton are constrained to diets of nanophytoplankton and strongly suggests that their grazing capability extends beyond boundaries assumed by size-based models. Dinoflagellates and oligotrich ciliates dominated the microzooplankton community. Estimates of abundance and biomass for microzooplankton >10 mm were higher than previously reported for the region, ranging from 22,000 to 227,430 cells l À1 and 18 to 164 mg C l À1 . Highest abundance and biomass occurred in the bloom and corresponded with increased abundance of the large ciliate Laboea, and the heterotrophic dinoflagellates Protoperidinium and Gyrodinium spp. Despite low grazing rates on phytoplankton o10 mm within the bloom, the abundance and biomass of small microzooplankton (o20 mm) capable of grazing E. huxleyi was relatively high at bloom stations. This body of evidence, coupled with observed high grazing rates on large phytoplankton cells, suggests the phytoplankton community composition was strongly regulated by herbivorous activity of microzooplankton. Because grazing behavior deviated from size-based model predictions and was not proportional to microzooplankton biomass, alternate mechanisms that dictate levels of grazing activity were in effect in the southeastern Bering Sea. We hypothesize that these mechanisms included morphological or chemical signaling between phytoplankton and micrograzers, which led to selective grazing pressure. r

Microzooplankton grazing rates on three phytoplankton size fractions (,5, 5-20, and .20 mm) were measured during spring and summer 2001 in the northern coastal Gulf of Alaska (CGOA). To a first approximation, microzooplankton consumed all production by phytoplankton ,20 mm in size and nearly half the production by phytoplankton .20 mm, mainly diatoms. Microzooplankton (ciliate plus heterotrophic dinoflagellate) biomass ranged from 9.6 mg C L 21 to 82.2 mg C L 21 . The highest levels were associated with diatom blooms and equaled those previously reported for highly productive coastal upwelling regions. Regulation of microzooplankton grazing differed according to size class. Grazing on phytoplankton ,5 mm in size averaged 0.48 d 21 and was closely correlated with phytoplankton growth rates in the same size class. In contrast, grazing on phytoplankton .20 mm averaged 0.17 d 21 and was unrelated to phytoplankton growth rate in this size class. Variations in grazing pressure on these largest phytoplankton arose mainly through variations in the biomass of the larger (.40 mm) ciliates and dinoflagellates. This biomass, in turn, became more closely correlated with .20 mm chlorophyll as the season progressed, indicating removal of top-down control on these ciliates and dinoflagellates as Neocalanus spp. copepods left the upper water column. Because microzooplankton directly consume much of the phytoplankton production in the CGOA, processes that regulate this trophic linkage have major implications for food web structure and secondary production in this coastal ecosystem.

The coastal Gulf of Alaska (CGOA) supports high abundances of invertebrates, fishes, and marine mammals. While variable from year to year, multi-decade fish production trends have been correlated with climate regimes such as the Pacific Decadal Oscillation. Winds, massive freshwater inputs, and complex topography in the CGOA create high-energy physical features on multiple time and space scales. This suggests that climate might be linked to higher trophic level production through the regulation of resources for primary producers. Data from spring and summer 2001 revealed seasonal and spatial variability in the factors regulating CGOA primary production. Some of the highest growth rates (>1.0 d -1 , as estimated with the seawater dilution technique) were measured in April diatom blooms. Nitrogen limitation of growth rates was evident as early as late April and appeared to follow closely the onset of spring stratification. The summer phytoplankton community was dominated by small (< 5 µm) cells exhibiting varying degrees of nitrogen limitation depending on cross-shelf location. However, we observed an intense mid-summer diatom bloom in the Alaska Coastal Current, perhaps a response to a series of upwelling events. Strong cross-shelf gradients governed every aspect of phytoplankton community structure and function, including overall biomass, cell size, species composition, nutrient utilization, growth rate, and degree of macronutrient limitation. These gradients were consistent with a cross-shelf gradient in dissolved iron availability. Because the type of resource limitation and the taxonomic composition of the phytoplankton community varied across the shelf, a stepwise regression of whole-shelf phytoplankton growth rates versus resource availability had little predictive power. The effect of climate-driven resource variation on primary production in the CGOA has to be understood in the context of different community types, their production potential, and the environmental conditions that dictate their extent and stability.

Several bloom‐forming marine algae produce concentrated intracellular dimethylsulfoniopropionate (DMSP) and display high DMSP cleavage activity in vitro and during lysis after grazing or viral attack. Here we show evidence for cleavage of DMSP in response to environmental cues among different strains of the haptophyte Emiliania huxleyi (Lohmann) Hay et Mohler and the dinoflagellate Alexandrium spp. (Halim). Sparging or shaking live cells of either taxon increased dimethyl sulfide (DMS), especially in dinoflagellates, known to be very sensitive to shear stresses. Additions of polyamines, known triggers of exocytosis in some protists, also stimulated DMSP cleavage in a dose‐responsive manner. We observed DMS production by some algae after shifts in light regime. When most exponential‐phase E. huxleyi were transferred to continuous darkness, cells decreased in volume and DMSP content within 24 h; DMSP content per unit cell volume remained relatively steady. DMS accumulated as long as cells remained in the dark, but on returning to a light:dark cycle DMS accumulation ceased within 24 h. However, E. huxleyi strain CCMP 373, containing highly active in vitro DMSP lyase, produced only transient accumulations of DMS in the dark. This was apparently due to production and concomitant oxidation or uptake of DMS, because cells of this strain rapidly removed DMS added to cultures. Three strains of the dinoflagellate Alexandrium tamarense containing high in vitro DMSP lyase activity showed no DMS production in the dark, and all appeared to remove additions of DMS. Alexandrium tamarense strain CCMP 1771 also removed dimethyl disulfide, an inhibitor of bacterial DMS consumption. These data suggest that physical or chemical cues can trigger algal DMSP cleavage, but DMS production may be masked by subsequent oxidation and/or uptake.