Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/104

Estuaries and Inshore Waters turn affect the movements of fish and their life cycles, because different fish have different salinity needs and tolerances.

98% of the sediment load discharged into the estuaries and inshore waters from the Niger Delta is quartz-sand; most of the silt borne down through the drainage system is trapped in the mangrove forests, and most of what escapes is then swept into the deep offshore canyons by long-shore drift.

Acidity (or pH), turbidity (cloudiness due to suspended sediment) and nutrient levels tend to be lower in estuaries fed by black-water and when river discharge is low.

High turbidity results in low light levels. Together with the low nutrient content of much of the discharged water, this results in a low primary production (i.e. of plant biomass), at between 100-150 mg per cubic metre (A.A. Amadi). However, overall bioactivity is high because of the relationship of the estuary/inshore waters ecosystem with the highly bioactive mangrove ecosystems adjacent to it. The simplest facet of this relationship is that many of the estuary and inshore animals feed on the plants and smaller animals of the mangrove ecosystem.

9.3 PLANTS AND ANIMALS OF THE ESTUARIES AND INSHORE WATERS

9.3.1 PLANKTON

Light-loving phytoplankton are primitive marine organisms that live by photosynthesis (see 4.5.3). Their activity tends to be low because turbidity results in low light penetration; however this is sometimes offset by the higher nutrient inputs during the wet season, when organic matter is discharged from the rivers.

Levels of the zooplankton which feed on the primary production of phytoplankton will naturally also be low.

9.3.2 SEAWEED

Seaweed may be red, green or brown algae. Turbidity also limits their presence and activity, but nitrogen-fixing oligotrophic blue-green algae are common. Together with detritus of mangrove forest systems, these are the basis of marine food chains.

9.3.3 INVERTEBRATES

Molluscs largely belong to the brackish-water ecosystems. However, although the Crustaceans (shrimps, primarily the Pink Shrimp Penaeus notali, crabs, and lobsters) all depend to some degree on mangrove detritus for food they properly belong to the estuary/inshore waters ecozone. 102