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Zooplankton community composition across a range of productivity regimes in coastal British Columbia Mahara, Natalie

Abstract

Coastal British Columbia is composed of deep channels and shallow sills intricately woven around a collection of small islands, creating complex oceanographic conditions. Consequently, seasonal production varies up to several orders of magnitude throughout the region, with some regions undergoing large annual phytoplankton blooms, while others have permanently low phytoplankton biomass. The region from the northern Strait of Georgia (SoG) to Johnstone Strait (JS) has been vastly understudied from the perspective of zooplankton despite its importance to many higher trophic levels. The objectives of this study were to: a) describe the annual seasonal cycle of zooplankton in the northern SoG over the course of two years and examine how communities differed, and b) examine the spatial patterns in zooplankton community composition and abundance from the northern SoG to JS, a region spanning seasonally productive / stratified areas (northern SoG, eastern Discovery Islands (DI)) as well as permanently high-nutrient, low-chlorophyll (HNLC) regions (JS, western DI). There was a strong signal of intra-annual seasonality in zooplankton community dynamics in the northern SoG, separating into distinct winter, early spring, and summer – fall assemblages. Despite a six week difference in spring bloom timing between years, peak zooplankton abundance occurred within the same calendar week during both years and community composition was similar between years, indicating that there is likely resilience within the zooplankton seasonal cycle to short scale interannual changes in environmental conditions. Our spatial analysis indicated that the northern SoG, DI, and JS each had distinct zooplankton communities with key species and densities separating regions. We detected minimal overlap in the JS zooplankton community with the DI and northern SoG, indicating that while DI zooplankton are likely sourced from the SoG, JS zooplankton likely originate from Queen Charlotte Sound. The DI were comprised of both productive and HNLC regions, yet zooplankton abundance was highest in the mixed western DI channels. Intense tidal mixing in the DI likely supplies nutrients to stratified surface waters in the eastern DI, which in turn supply zooplankton to the western DI with low in-situ production. These results provide insight into how feeding conditions for higher trophic levels may vary throughout the region.

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