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)