Hypoxia and Brittlestars: Linking Physiology and Behaviour to Ecosystem Function.

Hypoxic events are increasing in frequency and duration, particularly in areas susceptible to eutrophication. Such events pose a growing threat to the health and function of marine ecosystems by altering large-scale biological and ecological processes. Linking how hypoxia impacts upon key benthic invertebrates, both in terms of their physiology and behaviour, to the disruption of ecosystem function and services is of primary importance.

Station L4 forms part of the Western Channel Observatory, a long-standing oceanic time-series and marine biodiversity reference site. Recent observations indicate that L4 has experienced unusually large and long-lived spring blooms and seasonally driven periods of reduced O₂ levels altering benthic community composition. Using a mesocosm experiment, we investigated the effects of short-term moderate hypoxia (14 d, 3.59 mg O₂ L⁻¹) and organism density (5, 9, 13, 17, and 21 ind. per aquarium) on the aerobic metabolism and reproductive biology of a key infaunal species, the brittlestar Amphiura filiformis. In addition, bioturbation parameters were recorded as a measure of organism activity, alongside nutrient flux measurements as a proxy for ecosystem function effects.

Exposure to hypoxia resulted in reduced metabolism, and caused a delay in metabolic recovery rates once normoxic (8.09 mg O₂ L⁻¹) conditions were re-established. Additionally, hypoxia also caused a delay in females’ reproductive cell development, smaller oocyte diameters, and a greater number of pre-vitellogenic oocytes present within the ovaries. Interestingly, while organism density had no significant effect on the physiological or histological traits examined, it did have a positive effect on bioturbation activity, an effect which was reduced by hypoxia. The observed disruptions to metabolic rates, reproductive biology and organisms’ activity will be further discussed in terms of their ecological implications and possible long-term effects if repeated hypoxic events occur.