African Wildlife & Environment Issue 83

CONSERVATION

Umzimkhulu Estuary moderate flood with onshore wind

Depending on the prevailing wind in relation to the timing and intensity of the flood pulse, the sediment plume from the uMzimkhulu behaves in a manner that is fundamentally different from that of the Okavango. For example, a moderate onshore wind drives the sediment and nutrient plume out to sea, away from the immediate vicinity of the estuary. This can change dramatically in a relatively short space of time, if the wind should change direction. When the north easterly wind shifts slightly to a more onshore trajectory, it typically drives an inshore wedge northwards along the coast, visible at least 10 km distant to the estuary, while simultaneously driving the major portion of the plume along the coastline to the south. A slight change in the direction, and/or intensity of that prevailing north easterly wind, suddenly drives the plume in its entirety, away from the southern inshore region, out to deep water. These changes are rapid and unpredictable, with major implications for food web dynamics across a wide range of habitat, from close inshore benthic processes to deeper water beyond the surf zone. More importantly however, these

acquisition of prey rather than feel or smell.This is in direct contrast to the uMzimkhulu. The uMzimkhulu is a highly variable system when viewed strictly in terms of the interaction between just four variables – flood pulse, tidal pulse, nutrient load and wind direction / intensity. In direct contrast to the Okavango, the uMzimkhulu is relatively nutrient rich with a high silt load.This is derived from agricultural runoff upstream, as well as the discharge of sewage into the river at various points along its journey to the ocean.Technically we can consider the uMzimkhulu to be mesotrophic, given the elevated nutrient loads. The good news is that the system has not yet crossed the threshold into a eutrophic status, so widespread algal blooms are not yet a feature of the river. The silt plume is a major driving factor in all aquatic ecosystems downstream of the estuary. This means that the impact of the flood pulse in the uMzimkhulu River has a stochastic dimension to it. The term ‘stochastic’ refers to a random dimension that makes prediction more challenging and less reliable than in the case of the Okavango.

15 | African Wildlife & Environment | Issue 83 (2023)