In this first phase, we are taking advantage of pools that
remain isolated for several hours during low tide. The
pools are chosen by location, size, and degree of isolation. Ideally, a pool becomes completely isolated from
water flow in and out at low tide. At least one sample (but up
to five samples) is taken every 15 minutes until the pool is no
longer isolated due to the flooding tide. A water quality
instrument is placed in the pool throughout the experiment to
record water temperature, depth, dissolved oxygen, turbidity and
pH. Back in the lab, the water samples are analyzed for
ammonium, nitrate, phosphate, total nitrogen, total
phosphorous and total suspended sediments (organic and inorganic
materials). Different conditions are used per experiment
to understand the variation in nutrient levels due to MLO's.
For example, in one experiment we constructed a fence around
each pool and excluded nekton. In another experiment, extra
nekton were added to the pool to "spike" the number of fishes
within the pool. Experiments have also been conducted at
night, to determine nutrient levels when photosynthesis is not
occurring. By using various techniques to manipulate the
amount of nekton in the pool we can understand the contribution
of nekton towards the total amount of nutrients within the pool.
Along with taking water samples every 15 minutes during an
experiment, seines and lift nets are used at the conclusion of
each experiment to capture as many nekton as possible. The
nekton are then counted, weighed, and a subsample is taken to
approximate the length and weight distributions within the pool.
These data may help us make correlations concerning nekton size
and quantity and relationships to total nutrients and rate of
change of nutrient levels within the pool.
Pulling the
lift net at the conclusion of a pool experiment
Phase II (Summer
2006)
During
phase II of the study we are continuing to examine the effects
nekton have on intertidal pool nutrient concentrations within
natural and experimental intertidal pools. In addition to
our experiments, we are also monitoring selected creeks to
identify species composition, biomass, and species specific
growth rates of the major MLO species, which typically inhabit
intertidal creeks during summer months.
Small Mesocosm
Experiments
In addition to nekton, the water and sediments contain bacteria and other small
organisms that excrete nutrients into the water. To
quantify the effects that both sediment and nekton have on water
nutrient levels we are conducting a set of replicated
experiments using small mesocosms to mimic a natural tidal
pool while controlling the constituents of our experimental
pools. In these experiments small tubs are placed at the
bottom of an intertidal creek as the tide is ebbing. Equal
numbers of tubs are then randomly selected to contain one of
three treatments: nekton only, sediment only, nekton and
sediment, and control mesocosms with no nekton or sediment for
comparison of the treatments. As water continues to ebb,
the experimental mesocosms are allowed to become isolated
simulating a natural intertidal pool. Water samples are
taken at time of isolation and just before the mesocosms are
once again penetrated at the following flood tide. From
these measurements we can assess the difference in the amount
each treatment has contributed to the nutrients within a
mesocosm.
Large Mesocosm
(Laboratory) Experiments
We are also conducting a set of similar laboratory experiments
using a larger mesocosm designed to mimic a natural tidal creek.
One of the advantages of conducting laboratory experiments to
test ecological questions is that many variables naturally
inherent to nature can be controlled. For example, the use
of an "artificial" tidal creek allows us to control factors such
as water flow, volume, length of isolation, as well as the
abundance and biomass of nekton that are in our experimental
creek. Therefore, laboratory experiments are very valuable
in that they enable us to remove some of the variability in our
data which may inhibit us from detecting differences between our
treatments in a natural setting. Furthermore, laboratory
experiments help us confirm that the results that we are getting
from field experiments are truly due to the treatments and not
some unforeseeable variable. Our artificial tidal creek
consists of two separate sides, each with shallow, middle and
deep water levels. the mesocosm is then filled with
seawater. A known amount of nekton are then placed in one
side of the mesocosm while the other side is left absent of
nekton. Water quality measurements and samples are taken
at hour intervals for a set period of time and differences in
nutrient concentrations are compared between nekton and no
nekton sides of the mesocosm.
System-Wide Growth
Data
from previous studies in North Inlet, South Carolina have shown
that creek geomorphology is an important factor affecting
which creeks nekton may prefer to utilize. Several
factors, including the steepness of the bank, bottom type and
water velocity have been shown to be important to fish
utilization. If geomorphology is important factor then it
may be expected that fish may show a preference, or fidelity to
some creeks over others. Of course, for nekton to exhibit
fidelity to a particular habitat it may be hypothesized that
there is some benefit for
them to do so. One of our hypotheses is that due to the
geomorphology of the intertidal creeks, fishes who inhabit
creeks on a regular basis incur a selective advantage through
the availability resources and increased growth rates. In
an effort to examine this question we have been conducting site
fidelity experiments and system wide growth monitoring within
selected tidal creeks. To assess site fidelity we are
capturing nekton within specified locations and marking them
with a distinctive site specific marker such as a fin clip.
Nekton are then release and recsampled at various locations on a
later date. All nekton containing a mark are recorded to
see whether or not they were found in a different location.
Concurrent to our site fidelity work, we are also capturing
nekton from selected creeks over the entire growing season. Once
captured, nekton are identified and subamples are taken for
length and weight analysis. By-species length-weight
measurements from specific creeks over an entire season will
allow us to assess whether or not certain creeks tend to have
nekton with greater growth rates than others. A
possible connection of this study to our tidal creek nutrients
work is that we expect that if some creeks support greater
nekton growth rates (and greater relative biomass), then these
same creeks should also be greater contributors of nutrients to
the system.
Electronically measuring fish for length-weight analysis
Phase III
(Winter-Summer 2007)
During our 3rd and final phase of the project we will be using
the results obtained from the previous two years of the study to
broaden our scope on the contribution of nekton to nutrients
within North Inlet, South Carolina. There will be a continuation
of our field and laboratory mesocosm experiments during the
winter and summer months to see if there are differences in
nekton contribution by season. We will investigate our questions
regarding nekton site fidelity and how it relates to differences
in growth rates. In addition to growth, we will be aging
fish sampled during the 2006 season to see if fishes of
difference size were of the same age. If we can confirm
that fishes from our selected creeks are of similar age then
size differences may be due to differential resource
availability. Resource availability can affect fish growth
in several ways. As a result, we will also assess fish
condition through the analysis of total lipid content.
Condition data will give us information on whether or not fishes
from different creeks are just increasing in length or in total
stored fat as well. If some fishes are in fact "fatter" in
some creeks than others, then they may have a selective advantage
upon migration out of the estuary to inhabit offshore areas
during the winter where food is more scarce. During the winter,
after this next years new recruits have entered the estuary as
larvae and become postal-larval juveniles, we will begin
collecting these individuals to see what locations are inhabited
first and what size and age they are when they 1st enter the
creeks. We also intend
to look at the composition and distribution of benthic prey
within our spatial growth creeks.
