Ineffective artificial mouth-breaching practices and altered hydrology confound eutrophic symptoms in a temporarily closed estuary

Abstract

Context: Artificial breaching of intermittently closed estuaries has become more frequent in the face of global-change pressures. Aims: This study aimed to determine whether the ecological health of the Great Brak Estuary has been affected by the prolonged loss of marine connectivity arising from below-average inflow and failed breaching attempts. Methods: We characterised primary eutrophic symptoms (inorganic nutrients, dissolved oxygen, microalgae) typical of the various mouth states, i.e. open, closed and semi-closed. Key results: Initially, low inflow and closed mouth conditions facilitated the widespread occurrence of macroalgal blooms (Cladophora glomerata). Phytoplankton bloom conditions (>20 µg Chl-a L-1) ensued only in response to favourable hydrodynamic conditions (e.g. increased water residency, halocline formation) and increased nutrient availability from fluvial sources and macroalgal dieback. These blooms occurred in brackish conditions and comprised numerous taxa, including Cyclotella atomus var. marina, Cryptomonas sp. and Prorocentrum cordatum. Widespread hypoxia (<2 mg L-1) occurred during the semi-closed mouth phase because of the reduced flushing potential associated with the preceding high flow conditions. Conclusions: Global-change pressures and ineffective breaching practices will promote eutrophic conditions in intermittently closed estuaries in the future. Implications: Allocating sufficient environmental flows is key to preventing ecosystem degradation.