Seawater chemical gradients produced by heterotrophic ecosystem engineers

Ecosystem engineers modify physical aspects of an environment with implications for associated organisms. Often, this modification occurs through the dense association of many individuals, which, in marine systems, reduces water motion within the interstices of that aggregation. Reduced exchange of seawater surrounding these organisms may also limit the transfer of substrates or waste products of biological processes, leading to a depletion or accumulation of such compounds within biogenic habitats. In autotrophic, or photosynthetic, habitats, this may increase both oxygen and pH. In heterotrophic habitats lacking appreciable photosynthesis, however, modifications resulting from respiration and calcification can dominate which generally lower oxygen and pH, elevating stresses on both the habitat former and its residents. Here, we compare the chemistry modification capability of several ecosystem engineers including the California mussel and the purple sea urchin. In a laboratory flow tunnel, we measured vertical gradients of pH and oxygen above and within aggregations of each of these habitat-formers under a variety of seawater velocities. We observed decreases up to 0.1 pH and 25 µmol kg^-1 oxygen in the mussel bed. Gradients were much less pronounced within urchin aggregations with decreases of only 0.01 pH and 3 µmol kg^-1 oxygen observed. The strength of these gradients strongly depends on biological and physical contributions like respiration and calcification rates and seawater velocity. We additionally explore whether these gradients might impose feedbacks that alter the performance of the habitat former.