Effects of seasonal stratification on turbulent mixing in a hypereutrophic coastal lagoon

Abstract

Seasonal variations in vertical turbulent mixing rates, density structure, inorganic nutrient concentrations, and phytoplankton biomass were investigated in a shallow, tidally choked coastal lagoon. The project site, Rodeo Lagoon, is located in the Golden Gate National Recreation Area, California, and has experienced intense blooms of the cyanobacteria Nodularia spumigena and Microcystis aeruginosa in recent years. Monthly measurements collected along a transect of the lagoon from March 2006 to April 2008 show it is strongly stratified by brackish water in winter, when freshwater inputs from the watershed and saltwater inputs from storm surge are both at their highest. The squared buoyancy frequency exceeds 0.5 s 22 under these conditions. In summer, weaker diurnal temperature stratification is the result of strong light absorption characteristics in this hypereutrophic lagoon. Wind is the dominant driver of turbulent mixing. Although water depths of less than 2.5 m lead to the expectation of rapid vertical mixing, limited fetch and strong density gradients reduce the coupling of wind stress and bottom stress. The vertical turbulent diffusivity is reduced by as much as three orders of magnitude across the pycnocline, and the water column in and below the pycnocline shows active turbulence only intermittently. The annual cycle of salt-based stratification and accompanying reduction in turbulent exchange of nutrients between the sediments and overlying water column inhibit the flushing of nutrients out of the lagoon and contribute to excessive phytoplankton biomass.

Microbial Diversity and Cyanobacterial Production in Dziani Dzaha Crater Lake, a Unique Tropical Thalassohaline Environment

“…The high turbidity and low light penetration in Dziani Dzaha is typical of hypereutrophic ponds and lagoons [57] where resident phytoplankton and suspended detritic material absorb most of the incident light in the upper water layer. As a consequence, only the top meter of the lake is photosynthetically active.…”

Effects of light and nutrients on seasonal phytoplankton succession in a temperate eutrophic coastal lagoon

“…This suggests a degree of self-shading and a significant role for phytoplankton in distribution of light within the water column. In fact, a more extensive examination of chlorophyll a concentrations and light attenuation in Rodeo Lagoon from 2006 to 2008 indicated that chlorophyll a alone accounted for 73% of the observed variability in k d (Cousins et al, 2010). In 2007, a euphotic depth (the depth where PPFD is 1% of its surface value) as shallow as 0.5 m was frequently observed.…”

“…Thus, N. spumigena was a significant nitrogen contributor at the time of its proliferation but perhaps not significant in the overall yearly nutrient budget of the lagoon. The greater DIN source appears to be the sediments (Cousins et al, 2010), affecting algal succession in the bottom water.…”

“…Similarly, a 1-year sampling effort in 2006 by Silkie (2008) found a mean orthophosphate level of 2.4 lM and a mean ammonium level of 10 lM but did not detect NO x in the lagoon. High nutrient levels may reflect a large nutrient pool in the sediments that accumulate because density stratification limits scouring of the flocculent layer during winter rain events (Cousins et al, 2010).…”

Effects of Elevated Sea Levels and Waves on Southern California Estuaries During the 2015–2016 El Niño

“…Sustained high water in NB resulted in die-off of high marsh habitat that has been used previously as nesting habitat for several sensitive bird species (Dick Zembal, personal observations). If closures become more frequent, as they did during the El Niño conditions, the ICEs will experience an increased frequency of inundation of freshwater on saline habitats, hypoxic conditions (e.g., Gale et al 2006;Cousins et al 2010), prolonged periods of inundation at a fixed elevation, and would pose a greater risk to upstream flooding. More frequent inlet closures cause a shift from more saline marsh vegetation to more freshwater vegetation as the surface layer over the marsh is fairly fresh due to urban runoff (Los Peñasquitos Lagoon Foundation et al 2016).…”