Marine plankton

Abstract

Summary: The marine environment, which includes estuarine, coastal and open ocean waters, is a phylogenetically rich repository of planktonic cyanobacteria. All major cyanobacterial groups are represented in the marine plankton, yet specific environmental constraints strongly select for certain groups to dominate in geographically and climatically distinct regions of the world's oceans. In this chapter, physical, chemical and biotic properties of estuarine, coastal and open ocean habitats are examined with respect to their controls on the diversity, abundance and distributions of marine planktonic cyanobacteria. The focus is on the filamentous and colonial cyanobacteria that periodically accumulate as dense blooms that may discolor oceanic and coastal waters. Blooms are of considerable biogeochemical and ecological significance, because they represent large concentrations of phytoplankton biomass that impact carbon, nutrient (N, P, Fe and micronutrients), and oxygen cycling. The smaller coccoid picoplanktonic forms are an additionally important biomass fraction addressed elsewhere (see Chap. 20 by Scanlan). Marine planktonic cyanobacteria employ a suite of morphological, physiological and ecological adaptations and strategies aimed at optimizing growth and reproduction in response to environmental constraints, including nutrient depletion (oligotrophy), variable degrees of turbulence, sub-optimal light and temperature conditions that characterize much of the world's oceans. These include N2 fixation, nutrient sequestration and storage, buoyancy regulation, consortial and symbiotic associations, and coloniality. Specific planktonic taxa are able to exploit human and naturally-(climatic) induced environmental perturbations and changes, such as nutrient-enrichment, rising temperatures, increased tropical cyclone activity, altered rainfall patterns and droughts. Some cyanobacterial bloom taxa are considered harmful (CyanoHABs) because they can negatively affect water quality and habitat condition by producing toxins, exacerbating hypoxia, and altering food webs. Potential nutrient and other management strategies aimed at controlling CyanoHAB outbreaks and dominance are addressed. The extent and limits of biotic evolution in this ancient group of metabolically-diverse phototrophs has strongly affected the geochemical and biotic changes characterizing the evolution of the Earth's oceans and biosphere.