Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/103

Estuaries and Inshore Waters 9 THE NATURAL ESTUARIES AND IN-SHORE WATERS OF THE NIGER DELTA
 * Introduction
 * Ecosystem Dynamics of the Natural Estuaries and Inshore Waters
 * Plants and Animals of the Estuaries and Inshore Waters

9.1 INTRODUCTION

In comparison with land and rivers, very little is known about the Estuaries and In-shore Waters as an ecozone or as natural ecosystems. This is because they are far less accessible to human beings.

What we know about the ecozone is confined to what we learn of it from the shoreline; by passing over the top of it, as swimmers or in boats; by randomly pulling life out of it by fishing; by depth sounding and by very rare and clumsy deep-sea diving and dredging. This is like learning about the forest by skimming its canopy in a light plane and occasionally parachuting out.

This situation is especially difficult for ecologists. For example, depth sounding and dredging indicate the physical topography and geological makeup of the seabed, and may also allow a survey of species diversity of the seabed. However this tells us very little about how it works as a living system or of interactions with the wider environment, any more than a contour map and a bucket full of mixed soil horizons tells us about terrestrial soil surface systems.

The water of this ecozone has a salinity that is slightly less than offshore seawater. However salinity changes over a daily cycle, with the ebb and flow of the tides, and also seasonally according to rainfall, river flow and currents. Moreover, at any given time, salinity will be different in different parts of the ecozone: swimmers notice this as they swim from warm salty seawater to much cooler freshwater flowing into it, at low tide when the rivers flush out.

Mangroves form the prime vegetation of the estuaries and form an ecotone between the natural estuary/inshore water ecosystems and the natural freshwater ecosystems (see chapter 6.). Similarly, the estuaries/inshore waters ecosystems form an ecotone between the sea itself and the brackish-water ecosystems further inland.

The natural Estuaries and Inshore Waters ecozone covers roughly a quarter of the whole surface area of the Niger Delta. If the Delta is seen as the drainage 'sink' for much of Nigeria and other parts of West Africa, via the Niger River system, then the estuaries and inshore waters ecosystem is like a series of drainage holes through which everything passes on its way to the deeper sea.

9.2 ECOSYSTEM DYNAMICS OF THE ESTUARIES AND INSHORE WATERS

The physical environment varies over time and space in terms of salinity, acidity, nutrient status and light, to name just the major parameters.

For example, the salinity gradient from freshwater high up the estuary to seawater at or beyond the mouth changes over time. The gradient 'contour lines' are squeezed inland as seawater pushes up into the estuary with each high tide (there are two a day here); these contours also shift further out to sea during the annual wet season, when higher volumes of freshwater are coming downriver. These changes in 101