Type of Presentation

Oral

Session Title

Kelp and Eelgrass

Description

Echinoderm population cycles may be important drivers of ecological shifts on kelp bed communities. In Howe Sound, British Columbia, kelp beds are dominated the perennial Agarum fimbriatum, which provides important habitat for a variety of fish and invertebrates. The northeast Pacific recently experienced a mass mortality of sea stars beginning in early September 2013, and the sunflower star Pycnopodia helianthoides – a previously abundant predator of bottom-dwelling invertebrates –disappeared from many sites in a matter of weeks. By comparing the abundance of invertebrates and kelp before and after the sea star mortality event, we observed a four-fold increase in the abundance of green sea urchins and a 50% decline in kelp cover. Surveys were conducted during summer when Agarum blade size and growth is greatest. Here, we present the hypothesis that some of the changes we observed resulted from a trophic cascade involving sea stars, urchins and kelp. Behavioural experiments conducted in situ suggest predation release as a key mechanism underpinning the shift in urchin numbers. To quantify the impact of higher urchin abundance on kelp, we conducted 10 independent grazing experiments where A. fimbriatum was transplanted to barren rocky reefs of various green urchin densities. At high densities, kelp declined more that 40% in as little as 24 hrs. A. fimbriatum also provides key settlement habitat for the spot prawn Pandalus platyceros, so we predict the trophic cascade may impact spot prawn abundance in future years.

Keywords: algae, trophic cascade, echinoderm, sea star wasting, mass mortality event

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Kelp bed community shifts following sea star wasting syndrome in Howe Sound, British Columbia

2016SSEC

Echinoderm population cycles may be important drivers of ecological shifts on kelp bed communities. In Howe Sound, British Columbia, kelp beds are dominated the perennial Agarum fimbriatum, which provides important habitat for a variety of fish and invertebrates. The northeast Pacific recently experienced a mass mortality of sea stars beginning in early September 2013, and the sunflower star Pycnopodia helianthoides – a previously abundant predator of bottom-dwelling invertebrates –disappeared from many sites in a matter of weeks. By comparing the abundance of invertebrates and kelp before and after the sea star mortality event, we observed a four-fold increase in the abundance of green sea urchins and a 50% decline in kelp cover. Surveys were conducted during summer when Agarum blade size and growth is greatest. Here, we present the hypothesis that some of the changes we observed resulted from a trophic cascade involving sea stars, urchins and kelp. Behavioural experiments conducted in situ suggest predation release as a key mechanism underpinning the shift in urchin numbers. To quantify the impact of higher urchin abundance on kelp, we conducted 10 independent grazing experiments where A. fimbriatum was transplanted to barren rocky reefs of various green urchin densities. At high densities, kelp declined more that 40% in as little as 24 hrs. A. fimbriatum also provides key settlement habitat for the spot prawn Pandalus platyceros, so we predict the trophic cascade may impact spot prawn abundance in future years.

Keywords: algae, trophic cascade, echinoderm, sea star wasting, mass mortality event