The hypoxia that developed in a microtidal estuary following an extreme storm produced dramatic changes in the benthos

Abstract

Runoff from an extreme storm on 22 March 2010 led, during the next 3 months, to the formation of a pronounced halocline and underlying hypoxia in the upper reaches of the microtidal Swan–Canning Estuary. Benthic macroinvertebrates were sampled between January 2010 and October 2011 at five sites along 10km of this region. By mid-April, the number of species, total density, Simpson’s evenness index and taxonomic distinctness had declined markedly, crustaceans had disappeared and the densities of annelids and molluscs had declined slightly. These faunal attributes (except Simpson’s index) and species composition did not recover until after the end of the hypoxia. The survival of annelids and loss of crustaceans in this period reflects different sensitivities of these taxa to severe environmental stress. The results emphasise that microtidal estuaries with long residence times are highly vulnerable to the effects of environmental perturbations, particularly during warmer periods of the year.

Are floating algal mats a refuge from hypoxia for estuarine invertebrates?

“…Thus, only relatively mobile or hypoxia tolerant animals were ever sampled in this study. This differs from the results of Tweedley et al (2016) that showed a shift in the community from hypoxia sensitive to hypoxia tolerant species, i.e., a loss of crustaceans and relative increase of annelids, after storm-induced hypoxia.…”

“…Decomposition of algae further depresses dissolved oxygen, eventually resulting in hypoxia/anoxia and the development of toxic hydrogen sulphide and other stressors, e.g., ammonia (Valiela et al, 1997; Berezina, 2008). Sustained hypoxia or anoxia can have a catastrophic effect on the local faunal community with animals that are incapable of emigrating being at greatest risk (Ekau et al, 2010; Lyons et al, 2014; Tweedley et al, 2016). …”

“…While a variety of factors and mechanisms contributing to sustained hypoxia have been well-discussed previously (D’Avanzo & Kremer, 1994; Valiela et al, 1997; Touchette & Burkholder, 2000; Burkholder, Tomasko & Touchette, 2007; Heck & Valentine, 2007; Castro & Freitas, 2011; Cebrian, Corcoran & Lartigue, 2013), poor flushing (i.e., high water residence time) is a key factor leading to the development and persistence of hypoxia (Valiela, 1995; Valiela et al, 1997; Hemminga & Duarte, 2000; Burkholder, Tomasko & Touchette, 2007). Estuaries on the north and west coasts of PEI are microtidal and thus more susceptible to sustained hypoxia than estuaries on the south and east coasts that have greater tidal amplitude, although hypoxia can also occur there (Pingree & Griffithis, 1980; Bugden et al, 2014; Tweedley et al, 2016). …”

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

“…In recent decades, declining rainfall and river flows have reduced the frequency and scale of major flushing events (Silberstein et al ), encouraging increased water retention times and water column stratification (Hallett et al ). Hypoxia (i.e., dissolved O concentrations of <2 mgL −1 ) and harmful algal blooms are commonly associated with stratification events (Huang et al ) and are significant stressors of the system's ecology (Hallett, Valesini, Clarke et al ; Tweedley, Hallett et al ), periodically resulting in fish kills (Place et al ; Adolf et al ).…”

“…These periods of bottom‐water hypoxia were typically associated with water column stratification following freshwater flows and were particularly marked in the upper parts of the estuary and in deeper holes. Such hypoxia, which occurs as O is consumed during the microbial decomposition of organic matter, has long been an issue in the upper estuary, where stratification is more prevalent and the sediments are rich in nutrients and organic matter (Douglas et al ; Kurup and Hamilton ; Tweedley, Hallett et al ).…”