The influence of geomorphology and environmental conditions on stratification in Intermittently Open/Closed Estuaries

Abstract

Intermittently Open/Closed Estuaries (IOCE) have entrances that close during periods of low river flow. A major characteristic of IOCE is stratification of salinity, dissolved oxygen and water temperature. After IOCE open (either naturally or artificially), large changes in stratification occur as water drains from the estuary to the ocean. The rapid change in water level and loss of the top oxygenated layer of the water column during drainage often causes fish kills in IOCE globally and is related to stratification. Despite this, there are a lack of studies that statistically analyse the relationships between environmental variables and stratification and that quantify changes to stratification during the draining period across multiple IOCE. To fill these gaps, we (1) analysed the relationships between environmental variables and stratification using distance-based Linear Models (distLM) and distance-based Redundancy Analysis (dbRDA) in five different IOCE across Victoria, Australia, and (2) measured near-continuous physicochemical depth profiles and changes in entrance morphology, fluvial inflows, and estuary water level following four estuary openings.The distLM results revealed that maximum air temperature, mouth state and fluvial inflows showed statistically significant relationships with stratification in the IOCE studied. The dbRDA suggested that high maximum air temperatures were associated with low values of stratification in small IOCE, more commonly during closed periods. High fluvial inflows were associated with low values of stratification in large IOCE during open periods (except at one site, Curdies River). Field observations of changes in stratification during the draining period revealed two distinct responses. First, a high energy opening with discharge at the mouth between 70 and 182 m3s-1 and fluvial inflows of 0.87–1.85 m3s-1, causing the IOCE water column to mix and become uniform. Second, a low energy opening with discharge from the mouth between 6 and 37 m3s-1 and fluvial inflows of 0.02–0.03 m3s-1, causing the IOCE to remain stratified. These findings were summarised into a conceptual model showing the sequence of changes during openings for different types of IOCE. Over longer timescales (days to years), our results suggest that differences between stratification during open and closed periods are reflected at a shorter time-scale during the draining period (hours to days). These differences further reflect differences in geomorphology and hydrology between IOCE. Our findings will be useful for estuary managers to predict how stratification in different types of IOCE will change during artificial openings and provide a proxy for predicting their response to climate change.