Abiotic diel cycles and profiles in a bed of submerged plants in the coastal laguna de rocha (SW Atlantic)

Abstract

We determined the diel changes of abiotic profiles in the vegetated limnic littoral zone of the coastal Laguna de Rocha during summer days. Measurements were done in three sampling stations 40-55 cm deep every 2-3 hours and 5-10 cm. Submersed plants covered 100% of the sampling area and reached the surface of the water. Plant biomass was dominated by Eleocharis aff. nana (58%), Myriophyllum quitense (16%) and Nitella sp. (14%). Potamogeton pectinatus L., Zannichellia palustris L. and Chara sp. each represented less than 5% of total biomass. An evident diel cycle was observed for dissolved oxygen (1-12 mg L-1) and pH (6.5-8.0), which was probably induced by plant respiration and photosynthesis. Conversely, temperature (25-30 ºC) and conductivity (3.2 - 11.7 mS cm-1) showed some stratification favored by the presence of submerged vegetation, which impeded the circulation of water. The three sampling stations generally showed similar abiotic patterns despite their different plant composition and biomass. A short-term inverse stratification was produced by surface nocturnal cooling, where saline sea water lay below the river’s freshwater. We discuss the importance and variability of littoral processes in coastal lagoons both in time and depth, and their implications for the knowledge of coastal lagoons.