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Spatial and temporal variation in kelp-derived detritus and its dietary importance to consumers along the west coast of Vancouver Island, Canada Ramshaw, Brock Christopher

Abstract

Stable isotope analysis was used to determine spatial and temporal patterns of suspended kelp-derived detritus (KDD) and its contribution to consumers along a gradient of kelp abundance driven by recovering sea otter (Enhydra lutris) populations along the west coast of Vancouver Island (WCVI). During the summer and winter, ocean surface size-fractionated particulate organic matter (POM), dominant kelp species (order Laminariales), surface plankton and benthic organisms were sampled along offshore transects (0 – 30 km), and analyzed for carbon and nitrogen stable isotopes. Phytoplankton isotope fractionation characteristics were utilized along with principal component analysis to determine seasonally and size fraction specific δ¹³C and δ¹⁵N values. Blooming size fractions of phytoplankton were enriched in ¹³C by 2.5 to 5.4 ‰ and enriched by 0.3 to 3.1 ‰ in ¹⁵N. There were significant within and among region, and between season differences in kelp isotopic values. For example, during the summer, the otter-present Macrocystis pyrifera mean δ¹³C value (-13.17 ± 1.12 ‰) was more enriched than Nereocystis luetkeana (-16.89 ± 1.88 ‰; p < 0.001). These results were used in a Bayesian isotope mixing model (MixSIR) to estimate KDD contributions. The contribution to POM showed the greatest variability with distance from the kelp forest within the otter-absent region. However, in general, there was little difference among regions with respect to KDD contribution. Seasonally, KDD contribution to POM was greater during the summer. KDD contribution to total POM and 20 – 63 µm POM was similar and generally greater than 0.7 – 20 µm POM. Modeled estimates of KDD contribution to benthic invertebrates and fish were high (> 40 %) and similar with size, among regions and between seasons, with the exception of red turban snails (Astraea gibberosa) in the otter-present region where KDD contribution was 41.2 to 96.6 % lower than the otter-absent region. These results indicate that kelp abundance is not the only driver of KDD dispersal spatially and temporally. Factors such as local oceanography, kelp forest community composition, large variation in kelp isotope values, and similarity between kelp and phytoplankton isotope values may affect these patterns and lead to high uncertainty in modeled KDD contributions.

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