Influence of an Observed Decadal Decline in Wind Speed on Turbidity in the San Francisco Estuary

Abstract

Turbidity is an important habitat component in estuaries for many fishes and affects a range of other ecological functions. Decadal timescale declines in turbidity have been observed in the San Francisco Estuary (Estuary), with the declines generally attributed to a reduction in sediment supply to the Estuary and changes to the erodible sediment pool in the Estuary. However, we analyzed hourly wind data from 1995 through 2015 and found statistically significant declines of 13 to 48% in wind speed around the Estuary. This study applied a 3-D hydrodynamic, wave, and sediment transport model to evaluate the effects of the observed decrease in wind speed on turbidity in the Estuary. The reduction in wind speed over the past 20 years was predicted to result in a decrease in turbidity of 14 to 55% in Suisun Bay from October through January. These results highlight that the observed declines in both wind speed and sediment supply over the past 20 years have resulted in reduced turbidity in the San Francisco Estuary from October through January. This decline in turbidity in Suisun Bay potentially has negative effects on habitat for fish like the endangered Delta Smelt which are more commonly caught in relatively turbid water.

Nutrient Status of San Francisco Bay and Its Management Implications

“…These results show that the environmental setting of San Francisco Bay includes a wide range of estuarine environments along its two salinity gradients. Phytoplankton biomass (chl-a) is low relative to some other estuaries (Cloern and Jassby 2008). Exceptions occur as (usually shortlived) blooms.…”

“…These scenarios can be developed from California's Fourth Climate Change Assessment (Ackerly et al 2018) and its projections of: regional warming of up to 2.4°C by mid-century; more frequent, larger magnitude and longer duration heat waves (up to 40 more extreme heat days per year by 2050); increased magnitude of high precipitation events; and sea level rise potentially reaching 3 m by 2100. Equally important is the future trajectory of water clarity that has increased over recent decades because of altered wind patterns and reduced sediment supplies to the estuary (Bever et al 2018). Several 3D models of hydrodynamics and sediment transport have been calibrated and validated for San Francisco Bay (e.g., Elmilady et al 2018;Bever and MacWilliams 2013).These kinds of physical models can be coupled with nutrient-phytoplankton models to assess the likelihood that chl-a levels will regularly exceed numeric targets in a future climate.…”

Long-term changes on estuarine ciliates linked with modifications on wind patterns and water turbidity

“…Although the increase of water turbidity is generally attributed to an increase in wind speed (Bever et al 2018), in geomorphologically complex systems, other attributes such as the prevailing wind direction also come into play. The prominent role of wind direction on water turbidity revealed by the data, can be interpreted as the potential of winds cardinally aligned with the estuarine main channels to produce longer fetch and high energy wind…”

“…In flood-plain estuaries, wind-induced currents and wind waves are among the most important factors that favour sediment resuspension in the water column, and profoundly interfere with the light penetration (Weir and McManus 1987, Brand et al 2010, Bever et al 2018. A high concentration of suspended particles, in turn, can restrict ciliate reproduction (Jack and Gilbert 1993), while handling-limited predation by the interference of inedible suspensoids with prey uptake has been observed in planktonic filter feeders such as cladocerans (Laspoumaderes et al 2017).…”

Computational grid, subgrid, and pixels

“…Other inflows, pumps, and wind friction have been neglected but can be easily included to carry out specific and more realistic investigations. The interested reader may consult the recent literature (see, eg, other works [40][41][42][43][44][45][46] ) to access several interesting details of this complex study area.…”