Jennifer N Harding

PhD in Biology – Nutrient subsidies, food web dynamics and conservation


Sampling benthic stream invertebrates

Sampling benthic stream invertebrates

I am broadly interested in what structures communities and food webs.

My research focused on food web dynamics and trophic cascades in low order streams on the central coast of British Columbia.

I surveyed 16 different watersheds spanning 45 km near the Heiltsuk First Nation community of Bella Bella, BC, testing whether biofilm growth and invertebrate biomass and diversity are positively linked to salmon biomass.

My thesis explored how adult spawning salmon affect 1) algal and biofilm biomass, 2) benthic macroinvertebrate biomass, 3) benthic macroinvertebrate diversity and 4) food web structure in streams of the Pacific Northwest. I drew spatial comparisons across multiple streams, compared pre-spawn with post-peak spawning periods, and tested for
habitat characteristics that mediated these effects.

In Chapters 2 and 3, I showed that algal and invertebrate biomass in streams declined after salmon spawn, a likely result of substrate disturbance. Through the use of stable isotopes, I also showed that algae and invertebrates readily incorporated salmon-derived material. However, only algal biomass in the spring increased with salmon density. Spring invertebrate biomass was low in streams with high salmon densities, despite being enriched in both salmon-derived nitrogen and carbon. This may have been due to a slow recovery from the previous fall or in response to salmon nutrients subsidizing higher trophic levels and eliciting a trophic cascade.

Chapter 4 revealed that this decline in invertebrate biomass might have also been linked to life history traits. Invertebrate family composition shifted significantly across a gradient in salmon density and stream temperature, both prior to and post spawning. However, invertebrate family richness was not related to salmon density; it declined with
drainage size, a composite measure of stream and terrestrial habitat measurements.

Finally, Chapter 5 revealed that aquatic invertebrates and resident fish species in these coastal streams consumed a variety of resources throughout the year and that salmon were an important dietary source for many of these organisms, regardless of season.

My thesis demonstrated that salmon are a major structural component of coastal stream food webs, as a resource to benthic production, a major contributor to the diets of freshwater consumers, and as a source of disturbance that contributes to losses in lower trophic levels. Declines in wild Pacific salmon populations would significantly impact how coastal stream food webs function.

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