EAPSI: The Effect of Sea Temperature and Sediment Load on Respiration and Grazing on Kelp by Sea Urchins

Traiger Sarah B, Fairbanks AK

Investigators

Abstract

Kelp forests are stands of large seaweeds that provide important ecosystem services, such as providing habitat for fished species, contributing primary productivity to other habitats, and protecting coastal communities from waves and erosion. The persistence of kelp forests depends on the balance between kelp production and consumption by herbivores. Overgrazing by herbivores such as sea urchins can lead to deforestation and loss of kelp forest associated organisms. Ecklonia radiata is the main canopy-forming kelp in northeastern New Zealand and supports fisheries species such as spiny lobster. Sea temperature is predicted to increase over the 21st century both globally and in New Zealand waters. Previous research has shown that rate of food consumption by many marine invertebrates increases at elevated temperature due to increased metabolism. However, the effects of increased temperature on consumption rate may be affected by sedimentation, which is also predicted to change in the future. Sediment load to New Zealand waters has increased due to human activities, and weather changes and coastal erosion are predicted to increase sediment load in the future. This project will investigate the effects of future predicted temperature and sediment load on the grazing and respiration rate of the New Zealand sea urchin (Evechinus chloroticus). Grazing and respiration rates will be measured under current ambient conditions and predicted future elevated temperature and sediment load, separately and in combination, in a lab experiment conducted in collaboration with Dr. Nick Shears at the University of Auckland?s Leigh Marine Laboratory. Understanding the effects of environmental changes on this important member of the kelp forest community will help determine the future stability of kelp forests in New Zealand. Future changes in climate and coastal development could lead to high sediment conditions spreading into locations currently unaffected, with effects on trophic dynamics of those systems. The global stressor of increased sea temperature may interact with local stressors and alter top-down effects in kelp forest systems in ways that cannot be predicted by studying the effects of global or local stressors alone. In areas of low sedimentation, increased temperature could result in increased kelp consumption by urchins and reduced kelp biomass, or even shifts to urchin barrens. High sedimentation may increase urchin energetic costs by reducing grazing efficiency or causing urchins to increase movement. Temperature and sedimentation may have synergist negative effects on urchins, if grazing rates cannot increase to meet metabolic needs in the presence of sediment. This project will investigate the effects of increased temperature and sedimentation, separately and combined, on respiration and grazing rates of E. chloroticus on kelp (Ecklonia radiata) in a factorial lab experiment. The control treatment will be kept at ambient temperature (14°C) and sediment will be added to reflect the low concentrations that currently occur in New Zealand kelp forests (5 mg/cm2). The future temperature treatments will be 2°C above ambient, which is predicted to occur in the 21st century. High sediment treatments will reflect concentrations observed near Auckland where there is currently high terrestrial runoff (35 mg/cm2). The combined effects of temperature and sedimentation on respiration and grazing rates have not previously been tested, but may have critical consequences for trophic dynamics within kelp forests. The interaction between grazers and kelp is fundamental to the persistence of kelp forests, and the results of this experiment will help make predictions about future changes to kelp forests with climate change. This award, under the East Asia and Pacific Summer Institutes program, supports summer research by a U.S. graduate student and is jointly funded by NSF and the Royal Society of New Zealand.