QUANTITATIVE AND QUALITATIVE STUDIES OF ZOOPLANKTON IN IMO RIVER OF IMO STATE, NIGERIA

CHAPTER 1

INTRODUCTION

  • BACKGROUND INFORMATION ON ZOOPLANKTON

Zooplankton are heterotrophic (sometimes detritivorous) plankton. Plankton are organisms drifting in oceans, seas and bodies of freshwater. The word “zooplankton” is derived from the Greek zoon meaning “animals” and planktos meaning, meaning “wanderers” or “drifters” (Thurman,1998). Individual zooplankton are usually microscopic, but some (such as jellyfish) are large and visible to the naked eye. Zooplankton is a characterization spanning a range of organism sizes including small protozoans and large metazoans. It includes holoplanktonic organisms whose complete lifecycle lies within the plankton, as well as meroplanktonic organisms that spend part of their life in the plankton before graduating to either the nekton or sessile, benthic existence.  Although zooplankton are primarily transported ambient water currents, many have locomotion, used to avoid predators (as in diel vertical migration) or to increase prey rate (www.wikipedia.org//zooplankton).

Ecologically important protozoan zooplankton groups include the foraminiferans, radiolarians and dinoflagellates (the last of these are often mixotrophic). Important metazoan zooplankton include cnidarians such as jellyfish and the Portuguese Man o’ War; crustaceans such as copepods, ostracods, isopods, amphipods, mysids and krill; chaetognaths (arrow worms); molluscs such as pteropods, and chordates such as salps and juvenile fish. This wide phylogenetic range includes a similarly wide range in feeding behavior: filter feeding, predation and symbiosis with autotrophic phytoplankton as seen in corals. Zooplankton feed on bacterioplankton, phytoplankton, other zooplankton (sometimes cannibalistically), detritus (or marine snow) and even nektonic organisms. As a result, zooplankton are primarily found in surface waters where food resources (phytoplankton or other zooplankton) are abundant. Just as any species can be limited within a geographical region, so is zooplankton. However, species of zooplankton are not dispersed uniformly or randomly within a region of the ocean. Instead, ‘patches’ of zooplankton species (this also applies to phytoplankton) exist throughout the water body. Though few barriers exist above the mesopelagic, specific species of zooplankton are strictly restricted salinity and temperature gradients; while other species can withstand wide temperature and salinity gradients (Lalli and Parsons, 1993).

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