Since discovering that invertebrates contribute to nitrogen levels in the Southern California reefs and kelp forests throughout the summer and fall months, a team of UCSB scientists will conduct a study this summer to determine if the same can be said for sea otters.
This is a follow-up to a UCSB study published May 22 in the journal Global Change Biology, which determined that waste excreted by invertebrates like sea snails, limpets, lobsters and sea stars increases nitrogen levels in the ecosystem during the time of year that nutrients should be lacking.
Giant kelp can grow at a rate of 18 inches per day under ideal conditions. Although the upwelling that brings nutrients to Southern California’s ecosystem ceases around May, older studies by UCSB’s Marine Science Institute indicate that kelp continues to grow at ideal rates even after the currents stop delivering nutrients, according to a June 10 press release from UCSB.
Joey Peters, a UCSB Ph.D. student and the lead author of the Global Change Biology study, hypothesized that while invertebrates, particularly lobsters and sea stars, significantly contribute to kelp forest nitrogen levels through their excreted waste, those nutrients likely stay toward the seafloor. Because kelp gets most of its nutrients where a majority of its biomass is located, near the surface, Mr. Peters’ next study will explore whether other marine animals contribute to nitrogen levels in the kelp forest canopy.
Because sea otters forage for reef invertebrates and spend much of their time resting in the kelp forest, Mr. Peters suspects they act as “a conveyor belt of nutrients,” directly supplying their waste and urine into the canopy.
While the UCSB team’s collaborators at the United States Geological Survey have studied otter activity at monthly intervals for 13 years, Mr. Peters told the News-Press that he and his team will observe them on a “finer scale.”
That will entail recording how long the animals stay in the kelp forest canopy day to day, and using models to estimate how much they excrete. Whereas the invertebrate study covered five coastal locations, the upcoming study will focus on an area just south of Point Conception. Sea otter populations in many other Southern California waters are still lacking in numbers, so it was necessary to select an area with a consistently sizeable population to determine whether or not the creatures have a significant impact on nutrient levels.
“Sea otter populations are starting to recover in Southern California and that is a stable population,” Mr. Peters said.
In addition to observing sea otters, Mr. Peters will lead a study that takes a closer look at spiny lobsters and how their diet and the water temperature around them impact their nitrogen output.
According to the release, lobsters along with sea stars have an “outsized effect in shaping the ecosystem” because of their predatory nature. Because their diets contain more protein than other invertebrates, they release more ammonium and nitrogen-rich compounds in their waste.
The crustaceans gather together in dens during the day, which Mr. Peters suspects may result in more herbivores moving into the reefs if this leads to an increase in algae around the lobster dens.
Mr. Peters said he hopes to conclude the sea otter study by fall of this year.