Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi

Hydrobiologia, Oct 2017

We examined how freshwater flow and phytoplankton biomass affected abundance and population dynamics of the introduced subtropical copepod Pseudodiaptomus forbesi in brackish and freshwater regions of the San Francisco Estuary, California, USA. This copepod is key prey for the endangered and food-limited delta smelt, Hypomesus transpacificus, in low-salinity water during summer–autumn. Long-term monitoring data showed that P. forbesi was most abundant in fresh water, where summer–autumn abundance was invariant with freshwater flow. Abundance was positively related to freshwater flow in low-salinity water. Reproductive rates in both regions during 2010–2012 were low and unresponsive to chlorophyll or freshwater flow. Development indices, calculated as ratios of laboratory-derived to field-derived stage durations, were lowest for nauplii and highest for late copepodites, but averaged below 0.5 for all stages combined. Development indices were weakly related to chlorophyll for late copepodites only, unrelated to freshwater flow, and slightly higher in low-salinity than fresh water. Thus, the principal mechanism by which flow affects the P. forbesi population is apparently transport of copepods from fresh water to low-salinity water, where copepods are available to delta smelt. This work demonstrates how freshwater flow affects estuarine foodwebs through spatial subsidies of food supply.

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Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi

Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi Wim J. Kimmerer 0 1 Handling editor: Judit Padisa´k 0 1 0 K. R. Kayfetz Delta Science Program, Delta Stewardship Council , 980 Ninth St. Suite 1500, Sacramento, CA 95814 , USA 1 W. J. Kimmerer (&) T. R. Ignoffo A. M. Slaughter Romberg Tiburon Center for Environmental Studies, San Francisco State University , 3150 Paradise Dr., Tiburon, CA 94920-1205 , USA We examined how freshwater flow and phytoplankton biomass affected abundance and population dynamics of the introduced subtropical copepod Pseudodiaptomus forbesi in brackish and freshwater regions of the San Francisco Estuary, California, USA. This copepod is key prey for the endangered and food-limited delta smelt, Hypomesus transpacificus, in low-salinity water during summerautumn. Long-term monitoring data showed that P. forbesi was most abundant in fresh water, where summer-autumn abundance was invariant with freshwater flow. Abundance was positively related to freshwater flow in low-salinity water. Reproductive rates in both regions during 2010-2012 were low and unresponsive to chlorophyll or freshwater flow. Development indices, calculated as ratios of laboratory-derived to field-derived stage durations, were lowest for nauplii and highest for late copepodites, but averaged below 0.5 for all stages combined. Development indices were weakly related to chlorophyll for late copepodites only, unrelated to freshwater flow, and slightly higher in low-salinity than fresh water. Thus, the principal mechanism by which flow affects the P. forbesi population is apparently transport of copepods from fresh water to low-salinity water, where copepods are available to delta smelt. This work demonstrates how freshwater flow affects estuarine foodwebs through spatial subsidies of food supply. Pseudodiaptomus forbesi; Food limitation; Growth rate; Development pattern; Reproductive rate; Food webs Introduction Freshwater flow is a dominant influence on the state of estuaries. It can be the principal driver of interannual and seasonal variability in distributions of salinity and therefore biota, and can influence productivity at all trophic levels (Skreslet, 1986) . Climate change and increasing demand are expected to reduce the availability of fresh water to many estuaries, altering the magnitude and timing of fluctuations in flow and in these responses. Therefore, we need to understand better how variation in freshwater flow in estuaries influences physical properties and biological responses. Several impediments limit progress in understanding the mechanisms underlying biotic responses to freshwater flow. Numerous mechanisms potentially contribute to these responses, such as variation in nutrient loading, stratification, and predator-prey interactions (Drinkwater & Frank, 1994; Alber, 2002; Kimmerer, 2002) , and these may operate at different seasons and locations. At the landscape scale, the geomorphic, hydrologic, and biological complexity of most estuaries adds variability that may interfere with detection of mechanisms of change that are related to flow. For pelagic organisms which live in a moving frame of reference (Laprise & Dodson, 1993) , flow effects are probably best analyzed in a Lagrangian or moving frame of reference, which can be difficult in stratified estuaries and where mechanisms for flow effects have a geomorphic component. To overcome these impediments and determine mechanisms for flow effects on pelagic biota, investigations must determine how demographic processes of birth, development, mortality, and movement respond to flow and other environmental influences. For example, in a population that increases with increasing freshwater flow, a positive relationship of birth or growth rate to flow would suggest a mechanism related to food supply. Similarly, a negative relationship of mortality to flow might suggest that predation was reduced by high flow. Only by understanding how flow affects these processes, it is possible to interpret how abundance patterns vary with flow. In the San Francisco Estuary (SFE), annual abundance indices of several species of fish and one macroinvertebrate vary with freshwater flow (Jassby et al., 1995) . This variation appears to be a result of direct mechanisms rather than trophic effects because abundance of their zooplankton prey does not appear to vary with freshwater flow (Kimmerer, 2002; Kimmerer et al., 2013) . However, abundance alone is an incomplete measure of productivity, and little is known about how population dynamics of estuarine zooplankton responds to flow or to phytoplankton biomass, which may itself respond to flow (Drinkwater & Frank, 1994). This paper examines the abundance, egg production rate, development, and growth of the introduced calanoid copepod Pseudodiaptomus forbesi to variation in freshwater flow and phytoplankton bio (...truncated)


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Wim J. Kimmerer, Toni R. Ignoffo, Karen R. Kayfetz, Anne M. Slaughter. Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi, Hydrobiologia, 2017, pp. 1-18, DOI: 10.1007/s10750-017-3385-y