Studies on the biological oceanography of Haida eddies

Haida eddies are large (150— 300 km in diameter), long-lived (> 2 years), anticyclonic eddies that form each winter off the west coast of the Queen Charlotte Islands, in British Columbia, Canada (54°N, 130°W) and drift westward into the Gulf of Alaska, carrying nutrients and trace metals within their waters. This thesis documents the evolution of waters within an eddy called Haida-2000 over a 20-month period, focusing on nutrient and phytoplankton dynamics. Six surveys were completed over the course of the study and a second eddy (Haida- 2001) was sampled for comparison between eddies spawned in different years. Compared to coastal surroundings, the Haida-2000 eddy was rich in nitrate, phosphate, and silicic acid when it formed in early 2000. Once Haida-2000 moved offshore, however, it was nutrient-poor compared to High Nitrate Low Chlorophyll (HNLC) waters of the Alaska Gyre. New production rates from seasonal nitrate drawdown in the mixed layer fall between previously reported values at Ocean Station P (50°N, 145°W) and coastal values along the northern margin of the Gulf of Alaska. Silicic acid drawdown by phytoplankton exceeded nitrate drawdown by three times in the eddy's natal spring, during which time a large phytoplankton bloom was observed via satellite imagery. While nitrate drawdown rates were similar throughout the study, significant silicic acid drawdown was observed in the first year of eddy evolution only, signaling a shift from diatom-dominated coastal assemblage to a more oceanic one as the eddy aged. The large phytoplankton bloom observed in April 2000 within Haida-2000 was not repeated later in this eddy's evolution. Chlorophyll a biomass was higher within Haida eddies compared to surroundings during most of the sampling times, and the spatial distribution tended to follow isopycnals. The coherence of photosynthetic biomass with isopycnal structure suggests that the Haida-2000 and Haida-2001 eddies possessed convergent flow regimes near their edges, in contrast to a stronger, larger Haida eddy observed in 1998. It is most likely that frictional decay led to these patterns of convergent flow at the eddy edge in weaker eddies, while the stronger 1998 eddy exhibited divergent flow near its boundaries. Haida eddies were characterized by subsurface chlorophyll a maxima with the exception of uniform vertical profiles of chlorophyll a following strong storm-induced mixing in September 2001. Accounting for variability in spatial patterns of chlorophyll distribution improved estimates of maximum chlorophyll derived from surface chlorophyll concentrations. It was estimated that approximately 10% of daily primary productivity was grazed by planktonic ciliates. Based on abundance and biomass, this grazing pressure was relieved somewhat by the dilution of mixed layer waters by an intrusion of deep water during a period of vigorous mixing. Phytoplankton community composition was not dramatically different between Haida eddies and their surroundings during in Year 1 of their evolution. However, patterns ofsilicic acid drawdown in the first spring suggested that diatom growth was confined to the eddy and did not extend to surrounding waters. In Year 1, the phytoplankton assemblages at all sites were dominated by small representatives of coastal taxa, including Synechococcos spp., chlorophytes, cryptophytes, prasinophytes, euglenophytes and diatoms. In Year 2, larger differences in the phytoplankton assemblage were observed between eddy and reference waters, but all were dominated by more oceanic species, including haptophytes, pelagophytes, dinoflagellates, and chrysophytes. Key differences between eddy sites and their surroundings included a higher abundance of haptophyte algae at the eddy centre and the positive growth response to storm mixing by dinoflagellates at the eddy centre and diatoms in outside waters. Multivariate analyses showed that sampling sites first grouped together based on the time of sampling/distance from the coast, and then on the station type (centre, edge, or outside). Eddy edges and centre sites were more similar to each other in most instances than they were to outside reference sites. The use of microscopy to describe phytoplankton communites tended to lead to an overestimate of certain groups, particularly diatoms, due to the inclusion of weakly pigmented cells while chemotaxonomy did not. Signatures of offshore advection by eddy circulation were observed while Haida-2000 was close to the coast: the freshwater diatom Aulacoseira granulata was found at the eddy margin and in waters to the south, suggesting that delivery of terrestrial subsidies via this mechanism may be important to deep water organic material inventories. New observations of the calcareous dinoflagellates Thoracosphaera heimii occurring throughout this study may be related to water column stratification that has intensified in the Gulf of Alaska from the 1990's to the present. Rates of primary production were higher within eddy waters, and were negatively correlated with nitrate concentration, which was shown to be a reasonable proxy for dissolved iron content. Photosynthetic efficiency was highest at the eddy edges, and declined as iron concentrations decreased. Since nitrate had a negative relationship with iron concentration, it was inferred that the relationship observed between primary production and nitrate was due to iron limitation of phytoplankton photosynthesis. By including a term for nitrate in the Behrenfeld and Falkowski (1997) model of satellite-derived primary production, a better fit to actual data was achieved for the Gulf of Alaska. The smallest size class (0.2 - 5 jam) was responsible for an average of 68% of total primary production, with a proportionally larger contribution by this size class at the edges of Haida-2000 and Haida-2001. In order to examine the interplay between nitrogenous nutrition and iron concentrations, rates of nitrate and ammonium uptake were determined within Haida-2000 and Haida-2001. Rates of new production were highest in HNLC waters where concentrations of nitrate were high. When nitrate concentrations were low, new production rates were lowest, despite the presence of higher iron supply. Rates of new production were higher in Year 2 of eddy evolution when dissolved iron concentrations were lower than Year 1, suggesting that the relationships among nitrate utilization, iron concentrations, and export production rely on both nitrate and iron concentrations. Growth of plankton in mixed water masses that are typical of stirring at frontal regions was examined in a shipboard experiment. Drawdown of nitrate and phosphate were higher within waters composed of an equal proportion of eddy and outside water masses. Silicic acid drawdown was highest within eddy waters. Primary productivity and the accumulation of particulate carbon and nitrogen were also higher within the mixed water. Increases in chlorophyll a were not significantly higher within the mixed water, however, nor were any of the phytoplankton pigments. In the mixed water, the growth of Pseudo-nitzschia spp. exceeded that of the eddy water or outside water alone, suggesting a species-specific response to the new microenvironment. The timescale of the response to the mixing was on the order of the shift-up response documented in upwelling regions, and may explain spatial patterns of enhanced biomass observed toward the eastern side of eddies. The results from this thesis indicate that Haida eddies impart variability in the distribution and productivity of phytoplankton that is predictable. The differences between Haida eddies spawned in different years appeared to be related to differing dilution rates and eddy energies. Studying biological processes within anticyclonic eddies lends insight into ecosystem function in the Gulf of Alaska and should improve our ability to construct more accurate nutrient budgets, understand the factors that shape ecosystem structure in the northeast subarctic Pacific, and predict patterns of biological activity at the mesoscale.

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