Vertical mixing processes in Intermittently Closed and Open Lakes and Lagoons, and the dissolved oxygen response

Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are located on micro-tidal coasts (max. tidal range < 2 m) in temperate regions where the annual rainfall is non-seasonal. ICOLLs are generally shallow (<5 m depth) and are closed to the ocean due to the formation of an entrance bar for the majority of the year, when rainfalls are low. After periods of heavy rainfall, the super elevated water levels result in the natural or artificial breaching of the entrance bar. Due to their small size and absence of significant river inflows, ICOLLs exhibit strong temporal variations in their vertical density gradients, which can result in episodic density stratification. Such episodic stratification events may result in deterioration of the water quality including toxic algal blooms. This paper presents the results of field studies undertaken to determine the physical processes governing vertical mixing/ stratification in ICOLLs and their implications on dissolved oxygen dynamics. Data from two contrasting ICOLLs located along the southeastern coastline of Australia; (a) Wamberal Lagoon a small, shallower (w2 m max. depth) frequently open ICOLL; and, (2) Smiths Lake, a larger, deeper (w5 m max. depth) infrequently open ICOLL, are presented. The results indicated that Wamberal Lagoon was susceptible to periods of stratification during both the closed and the open states. During the closed state, periods of rainfall, low wind and/or high solar insolation led to short (<3 days) and irregular stratification events, whilst during the open state, stratification events occurred through a combination of rainfall, low winds and variations in tidal mixing. There was a tendency for dissolved oxygen to decrease, in the bottom waters, when the Buoyancy Frequency was >0.1 s 1. Smiths Lake demonstrated higher vertical stability and exhibited a tendency for persistent stratification, during both the closed and open states, primarily due to solar insolation (closed state) and gravitational circulation (open state), respectively. The persistent stratification maintained a vertical gradient in dissolved oxygen between the surface and bottom layers. However, tidal pumping associated with fortnightly tides appears to promote isolation of the bottom waters, causing the dissolved oxygen rates to temporarily decrease (for approximately 5 days) during the neap cycle.