Processes driving circulation, exchange and flushing within intermittently closing and opening lakes and lagoons

Abstract

The circulation and exchange between two intermittently closing and opening lakes and lagoons (ICOLLs) and the ocean were analysed using salinity and current meter data. Wamberal Lagoon was shallow (~2.5 m maximum depth) with a small (<1 km2) waterway area and a short opening duration (2 weeks), and Smiths Lake was deeper (~5 m maximum depth) with a larger (~11 km2) waterway area and a longer opening duration (4 months). An absence of river inflow and a restricted sill type inlet channel characterised both systems. The results showed that the smaller ICOLL exhibited a salinity structure similar to a partially or well-mixed estuary, whereas the larger ICOLL exhibited stronger separation of flow with a salt wedge-type structure. Both ICOLLs had strong current velocities in the surface water during ebb tide. The results also demonstrated that tidal effects controlled the circulation and exchange in the smaller ICOLL, whilst the wind also had a significant influence. In the larger ICOLL, subtidal effects over the fortnightly tidal cycle significantly influenced the circulation and exchange, but the influence of tide and wind effects was weaker. An analysis of the flushing for the two ICOLLs illustrated that the timescales were an order of magnitude different but comparable to the duration of opening for each ICOLL, ranging from 4 days in the smaller ICOLL (Wamberal Lagoon) to 113 days in the larger ICOLL (Smiths Lake). This research suggests that intermittent estuarine systems may be quite complex and variable within the same regional area, and consequently throughout the world.