Fish Kick Up Nutrients

Ocean plants – algae and phytoplankton – depend on nutrients for photosysnthesis. These nutrients – especially phosphorus – are often lost from the photic zone when they sink to the seafloor. Some loss is recovered by chemical diffusion and resuspension by currents. We know that fish activity can stir up nutrients in shallow lakes, but their role in the marine realm was not known. While surveying the Saanich site for VENUS node deployment, we encountered a large population of flatfish (the Pacific slender sole, Lyopsetta exilis) that created “puffs” of sediment as they swam.

Passive acoustic and optical instruments on the VENUS instrument platform quantified the resuspension activity of the fish at the bottom of Saanich Inlet. Sensors recorded 50 to 100 resuspension events per square metre per day!

Later, ROPOS (the remotely operated vehicle) surveyed the fish and used water samplers to sample the resuspended sediment stirred up as the fish swam.

These samples indicate that fish inject quantities of nutrients into the overlying waters. Our calculations suggest that benthic fish activity increases the immediate nutrient concentration in the water – ammonia 166%, phosphate 53%, and silica 3% higher than fluxes from undisturbed surface sediments.

The constant reworking of the sediment by fish is potentially an important, yet overlooked, contributor to marine geochemical cycles, enhancing nutrient and organic matter recycling and reducing the rate of carbon burial and digenesis processes. VENUS cameras will continue to examine fish activity in 2006.

Research by Ruthy Yahel with collaborators Gitai Yahel and Verena Tunniclife at University of Victoria; and Timor Katz, Boaz Lazar, and Barak Herut at Hebrew University.

Sample video:

Clip 1: VENUS CMAP camera records Lyposetta exilis stirring up sediment. March 30th, 2006, Saanich Inlet CMAP camera.

Clip 2: VENUS CMAP camera records Lyposetta exilis stirring up sediment. April 11th, 2006, Saanich Inlet CMAP camera.