Multi-nutrient limitation in the Coastal Gulf of Alaska: seasonal and interannual variability

Jerome Fiechter, UC Santa Cruz; Andrew M. Moore, UC Santa Cruz; Hernan G. Arango, Rutgers University.

Nutrient limitation and regional ecosystem dynamics in the northwestern Coastal Gulf of Alaska (CGOA) on monthly to inter-annual timescales is investigated by coupling a lower trophic ecosystem model to a three-dimensional coastal ocean circulation model. By including explicit growth limitation by light, nitrate, ammonium, silicate, and iron, the ecosystem model provides an ideal framework to investigate the combined role of macro- and micro-nutrients in shaping phytoplankton community structure. Based on comparisons with available in situ and remotely-sensed observations from 1998 to 2002, the model reproduces the dominant modes of variability associated with the northwestern CGOA ecosystem dynamics. Empirical orthogonal functions (EOFs) for nutrient limitation indicate alongshelf variations in diatom growth regime, with the northeast subregion limited mainly by nitrate, and the southwest subregion limited by both nitrate and silicate. At the shelfbreak, iron limitation regulates diatom growth. Changes in dissolved iron availability in that region will likely shift the cross-shelf phytoplankton community structure between the diatom-dominated shelf population and nanophytoplankton-dominated basin population. Furthermore, the EOFs suggest that regions where nitrate, silicate, and iron most severely limit phytoplankton growth vary in time. This implies that not only the frequency but also the timing of cross-shelf transport events will affect the seasonal and inter-annual CGOA ecosystem variability.