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Marine Ecology Progress Series

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MEPS 328:75-92 (2006)  -  doi:10.3354/meps328075

Cross-shelf gradients in phytoplankton community structure, nutrient utilization, and growth rate in the coastal Gulf of Alaska

Suzanne L. Strom1,*, M. Brady Olson1, Erin L. Macri1, Calvin W. Mordy2

1Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Road, Anacortes, Washington 98221, USA
2Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Seattle, Washington 98195-4235, USA

ABSTRACT: 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.

KEY WORDS: Phytoplankton · Chlorophyll size class · Nitrogen · Iron · Growth regulation · Bloom · Continental shelf

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