Long-Term Trends in Summertime Habitat Suitability for Delta Smelt, &lt;em&gt;Hypomesus transpacificus&lt;/em&gt;

We measured primary production during springsummer 2006-2007 to determine the carbon supply to the low-salinity pelagic food web of the San Francisco Estuary (SFE). Weekly or biweekly samples were taken at three stations of fixed salinity for size-fractionated primary production and biomass, both as chlorophyll and from biovolume based on counts. Error variance in productivity estimates arose mainly from the depth integration of 14 C uptake, showing the importance of productivity measurements at high light levels for estimates of depth-integrated production. Temporal and spatial variability in production were surprisingly small. Combining data from this study with long-term monitoring data, productivity and biomass were variable in time and salinity but without persistent patterns and with infrequent blooms. Production within the lowsalinity zone was unresponsive to variation in freshwater flow, in contrast to findings in other estuaries where nutrient loading drives variability in production and other regions of the SFE where production responds to residence time or to stratification. Estimated annual primary production was only 25 and 31 gC m −2 year −1 during 2006 and 2007, only half of it in cells >5 μm. These results imply that phytoplankton provided poor food web support for higher trophic levels, probably contributing to the long-term decline in fish abundance in the brackish to freshwater region of the estuary.

Section: Temporal Patternsmentioning

confidence: 97%

Short-Term and Interannual Variability in Primary Production in the Low-Salinity Zone of the San Francisco Estuary

Kimmerer

Parker

Lidström

et al. 2012

“…However, in systems where SSC is the primary driver of turbidity, conversion curves can be used to convert between turbidity and SSC. Due to the large number of available turbidity observations, and the widespread use of turbidity (rather than SSC) as a habitat indicator for many species, including Delta Smelt (e.g., Nobriga et al 2008;Sommer and Mejia 2013;Bever et al 2016), the model predictions of SSC were converted to turbidity using conversion curves developed from field observations. This allows for the inclusion of the physical processes driving the sediment transport and SSC into predictions of turbidity.…”

Section: Conversion Of Modeled Suspended Sediment Concentration To Tumentioning

confidence: 99%

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

Bever¹,

MacWilliams²,

Fullerton³

2018

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.

“…Increasing water clarity has been attributed as one potential cause of multiple subsequent ecologic impacts in the Delta, including increased harmful algal blooms (Jassby et al 2003;Glibert et al 2014), persistent SAV distribution favoring invasive species (Santos et al 2009(Santos et al , 2012, and decreased fish abundances . Multiple modeling studies have identified water clarity as an important factor in determining the abundances of the declining pelagic fish species including delta smelt, longfin smelt, or striped bass (e.g., Feyrer et al 2007;Mac Nally et al 2010;Nobriga et al 2008;Thomson et al 2010). These studies identify direct and indirect effects of increasing water clarity in the Delta.…”

Section: Management Implicationsmentioning

confidence: 99%

The Effect of Submerged Aquatic Vegetation Expansion on a Declining Turbidity Trend in the Sacramento-San Joaquin River Delta

Hestir

Schoellhamer

Greenberg

et al. 2015

Submerged aquatic vegetation (SAV) has welldocumented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAVon the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from longterm monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21-70 % of the total declining turbidity trend is due to SAV expansion.