Fig. 14.10 Left: two-dimensional principal-component analysis locating sampling
stations around Gisborne outfall in species “space”. Right: PCA plot overlaid with the
sediment oil and grease content at each sampling location. Sediment community
composition close to the outfall is distinct, clearly a response to deposits from sewage.
(After Roper et al. 1989.)
(^) Estuaries provide the strongest horizontal gradients in habitat conditions of any
marine habitat, save for the intertidal. As fresh water flows in from the upstream river,
it tends to ride over water entering from the ocean downstream and moving landward
beneath the outflow as a “salt wedge”. The vertical salinity gradient between the
surface layer and salt wedge can be sharp if freshwater flow is strong and tidal mixing
is weak. Conversely, most of the gradient can be along the horizontal axis of the