Si availability, phytoplankton species composition, and eutrophication

Quay Dortch and Nancy N. Rabalais and ¹R.E. Turner

Louisiana Universities Marine Consortium
8124 Highway 56
Chauvin, Louisiana 70344
qdortch@lumcon.edu
nrabalais@lumcon.edu

¹Coastal Ecology Institute and Department of Oceanography and Coastal Science
Louisiana State University
Baton Rouge, LA 70803
euturne@lsu.edu

Most studies of eutrophication focus on the consequences of increasing inputs of N and P in coastal waters but ignore decreasing or stable Si inputs. While increasing N and P are the cause of many of the harmful effects associated with increasing eutrophication, the changes in Si relative to N and P availability may have a profound effect on the impacts of eutrophication. The most obvious is the switch from diatoms to non-diatoms (see Turner et al.). Diatoms are usually associated with greater vertical carbon flux, either directly due to sinking cells or indirectly due to enhanced zooplankton grazing and production of sinking fecal pellets, both of which contribute to the formation of hypoxic bottom water. Non-diatoms may not contribute as much to vertical carbon flux as diatoms. In addition since most Harmful Algal Bloom species are non-diatoms, HABs have been hypothesised to increase when Si is limiting.

In reality the situation is much more complex because the response is often dependent on the individual species that dominate. Some ecosystems are either intermittently or borderline Si limited which allows diatoms with low Si requirements to dominate, rather than resulting in a shift to non-diatoms. Not all diatoms contribute equally to carbon flux and this may be determined in part by Si availability. Further, diatoms can be toxic or harmful, for example, some Pseudo-nitzschia spp. or Chaetoceros spp., and the dominance of some non-diatom HABs, such as colonial cyanobacteria, appear often to be independent of Si availability. Finally, for both diatoms and non-diatoms, other environmental factors may interact with silicate availability to determine which species predominate. In future research much greater emphasis needs to be placed on the role of individual species in determining the impacts of eutrophication and the role of Si availability should always be evaluated.