The Contrasting Ecology of Temperate Macrotidal and Microtidal Estuaries

Abstract

Tidal range is a master factor governing the differences in physico-chemical and biological characteristics between microtidal (<2 m) and macrotidal (>2 m) estuaries, which, for convenience, thus include mesotidal estuaries (2-4 m). Microtidal estuaries differ from macrotidal estuaries in geomorphology, tidal water movements, salinity regimes, residence times, turbidity, sedimentology and intertidal area. Consequently, their phytoplankton, microphytobenthos and macrophytes communities differ in biomass and production, areal extent, distribution patterns and composition. Mesozooplankton comprise predominantly autochthonous species in microtidal estuaries and allochthonous species in macrotidal estuaries. Meiobenthos in microtidal estuaries have greater densities in subtidal than intertidal areas and species persist along the estuary. Macrobenthos is dominated by small depositfeeding species in microtidal estuaries, whereas macrotidal estuaries contain some larger species and suspension feeders. Species richness and abundance of estuarine-resident fish species and the contributions of diving piscivorous birds and wading invertebrate-feeding birds are greater in microtidal estuaries. As paradigms regarding estuarine ecology have been based mainly on northern hemisphere macrotidal systems, this review has redressed this imbalance by detailing the extent of differences between microtidal and macrotidal estuaries. In particular, it uses data and case studies for southern hemisphere microtidal systems to demonstrate that the physico-chemical characteristics and ecology of the main flora and fauna of microtidal estuaries are frequently not consistent with those paradigms.

Baseline survey of the fish fauna of a highly eutrophic estuary and evidence for its colonisation by Goldfish (Carassius auratus)

“…Broke, Wilson and Irwin inlets; 11-20 species), which like the Vasse-Wonnerup, all maintain a seasonal connection to the ocean ). The relatively depauperate nature of the fish fauna of the Vasse and Wonnerup estuaries reflects the fact that the bar at the mouth of Wonnerup Inlet is only open to the ocean for part of the year, thus reducing the opportunity for marine species to penetrate the system (Strydom 2015;Tweedley et al 2016), and also the fact that, although this study was undertaken during a period when the bar at the mouth of Wonnerup Inlet was open, the movements of marine species into the estuaries are inhibited by the surge barriers (Fig. 1c).…”

“…Microtidal systems, i.e. those with a tidal range \2 m, located in Mediterranean climate regions are particularly susceptible to degradation, due to their limited tidal water movement, highly seasonal rainfall and, in some cases, ephemeral connection to the ocean (Potter et al 2010;Tweedley et al 2016). These characteristics increase the residency time of water in the estuary and facilitate the trapping of fine sediments and contaminants (Patchineelam et al 1999;Tweedley et al 2016).…”

“…those with a tidal range \2 m, located in Mediterranean climate regions are particularly susceptible to degradation, due to their limited tidal water movement, highly seasonal rainfall and, in some cases, ephemeral connection to the ocean (Potter et al 2010;Tweedley et al 2016). These characteristics increase the residency time of water in the estuary and facilitate the trapping of fine sediments and contaminants (Patchineelam et al 1999;Tweedley et al 2016). Given the 'natural susceptibility' of microtidal estuaries, such as those in south-western Australia, to environmental degradation and the effects on these systems of increased urbanisation and eutrophication, it is unsurprising that the environmental health of some of these systems has been shown to have declined since the 1980s Tweedley et al 2012Tweedley et al , 2014.…”

The Fish Community Index: A Practical Management Tool for Monitoring and Reporting Estuarine Ecological Condition

“…Moreover, several existing biotic indices from the northern hemisphere have proven unsuitable for application to Australian estuaries because natural disturbances, such as the more extreme hydrological and physicochemical variability of Australia's estuaries, can mask signals of anthropogenic impact (Warwick et al ). For example, the benthic macroinvertebrate faunas of southwestern Australia (SWA) estuaries are dominated by tolerant, opportunist taxa, reflecting both the extremely variable salinities (Van Diggelen and Montagna ) and natural accumulations of organic matter in these systems with long residence times (Tweedley, Warwick et al ). This precludes the application of the globally recognized, species‐level AZTI Marine Biotic Index (AMBI) for assessing the condition of estuaries in SWA (Tweedley et al ).…”

Growth, condition, and maturity schedules of an estuarine fish species change in estuaries following increased hypoxia due to climate change

“…During recent decades, the hydrological characteristics of these important systems (and coastal seas) have become modified in many parts of the world as a result of changes in rainfall and other effects of climate change (Altieri & Gedan, ; Hewitt, Ellis, & Thrush, ). For example, marked reductions in precipitation have led to declines in discharge and thus the effectiveness of flushing (Altieri & Gedan, ; Tweedley, Warwick, & Potter, ). Such declines, in turn, increase residence time and thus the retention of nutrients and organic material and, consequently, increases in primary productivity, which can ultimately lead to marked eutrophication and the development of oxygen‐depleted “dead” zones (Diaz & Rosenberg, ; Thronson & Quigg, ).…”