INTRODUCTION
{Note to students: be sure that all references below appear in your bibliography. You should also
use at least three (one can be the textbook) in your discussion.}
The abiotic environment is an important determinant of species distribution and abundance
in the natural world. However, it can be difficult to determine which component of the
environment is most important in determining distribution for particular species or types of
organisms, because many physical factors can potentially be critical for survival and reproduction.
All organisms must maintain their cellular solute concentrations within some range for
metabolic processes to continue. Since the solute concentrations in most organisms differ from
that of their external environment, organisms must typically expend energy to compensate for
osmotic water movement (Molles 1999). Furthermore, maintaining homeostatic solute
concentrations can be a challenge if the osmolarity of the external environment varies. In estuarine
environments, salinity may change rapidly due to rain or surface runoff. For marine organisms
that are acclimated to the salinity of seawater, encountering the low salinity of estuaries can cause
stress or death, depending on the tolerance limits of the organism. Those species that are more
tolerant of low salinity may be able to acclimatize, allowing them to persist in those environments.
For these reasons, one finds that as salinity decreases, the number of species present also decreases
until the salinity level drops enough that freshwater species can begin colonizing the area
(Vernberg and Vernberg 1972, Lockwood 1976). Also it has been found that distributions for
some species within an estuary strongly correlate with their relative tolerance to low salinity
(Vernberg and Vernberg 1972).
Lockwood (1976) suggests three ways that organisms deal with the stress of low salinity.
First, they detect and avoid changes in salinity. For example, they might swim to deeper water to
avoid a surface layer of brackish water, or avoid swimming into estuaries where the water can be at
lower salinity than in the ocean. Second they may be able to tolerate extreme conditions long
enough for more favorable conditions to return. Finally, there are physiological and
morphological adaptations the organism can make in order to survive in a hypoosmotic
environment. These adaptations are summarized by Maetz (1974) as 1) reducing body surface
permeability to water and salts, and 2) developing active mechanisms to uptake salts and reabsorb
them from the excretory system.
Osmotic challenges are not the only difficulties imposed by the physical environment.
Other factors that influence the ability of organisms to occupy a particular environment include
their tolerance limits for temperature, pH, light levels, and oxygen levels, to name a few (Molles
2001).
In this study, we examined the distribution and abundance of two zooplankton species in
three different habitats near Bellingham Bay. The first was in the Bay itself, the second was in the
estuary at the outlet of Padden Creek into the Bay, and the third was in Padden Creek. In addition,
we examined many aspects of the physical environment to determine which factors vary among
these three sites. We sought to test the following hypotheses: 1) zooplankton abundance will be
similar in all habitats examined (null hypothesis); and 2) if differences in abundance are seen,
differences in abiotic variables will explain the abundance differences.
METHODS
(Note to students: fill in the appropriate numbers from the measurements that you did wherever
there are X’s or ?’s below. Also, some of the methods have changed slightly from this earlier
protocol – be sure you fix this section to match what you actually did.)
We sampled three habitats for zooplankton abundance and abiotic variables. In Bellingham Bay,
we sampled from the boat launch dock on the south side of the Padden Creek outlet. We used a
Hydrolab to measure water salinity, pH, and temperature, and a light meter to measure solar
radiation, all at one meter depth. We used a plankton net (333 mesh) to sample zooplankton, by
pulling the net attached to a pole held at 1 m. depth back and forth along the dock for a total of ???
meters (total water volume sampled = ???). The estuary at the outlet of Padden Creek was sampled
from shore by pulling the plankton net for a total of ??? m (total water volume sampled = ???).
{Due to shallow water, we sampled the surface 50 cm rather than at 1 m as in the Bay. Hydrolab
and light measurements were made at 0.4 m. Note: this has been done in the past. Make sure to
include or not in your methods depending on what we actually did.} In Padden Creek, we sampled
for zooplankton approximately 30m inland from the bridge at Harris St.. We first calculated
stream flow velocity, then left the plankton net in place in the stream for X min. to allow a
sampling of X m of stream water (total volume = ???). Hydrolab and light measurements were
made at X m depth in the stream. For all sites, the contents of the plankton net were rinsed into
separate jars for counting in the lab.
After observing our samples, we decided to count two types of organisms: copepods and
larvaceans. All organisms were counted in a 50 ml subsample from each plankton tow according
to the methods outlined in Hooper and Peterson (2002). Estimates of the total number of
individuals of each species taken in the plankton tows were based their abundance in these
subsamples. Class data were then pooled to obtain averages and standard deviations for
abundances of each type of organism and for each of the physical parameters.
References
Lockwood, A.P.M. 1976. Physiological adaptation to life in estuaries. pp. 315-369 in Newell,
R.C., ed. Adaptation to environment: essays on the physiology of marine animals.
Butterworths, London.
Maetz, J. 1974. Aspects of adaptation to hypo-osmotic and hyper-osmotic environments. pp. 1-167
in Malins, D.C. and Sargent, J.R., eds. Biochemical and biophysical perspectives in marine
biology. Academic Press, London.
Molles, M. 2002. Ecology: Concepts and Applications. McGraw-Hill, NY, NY.
Peterson, M., and D. Hooper. 2002. Ecology Lab Manual, Western Washington University,
Bellingham, WA.
Vernberg, W.B. and F.J. Vernberg. 1972. Environmental physiology of marine animals.
Springer-Verlag, NY, NY.