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which could not be identified in the field were preserved and returned
to the laboratory for identification. Samples of blue-green algae were
also collected for laboratory identification, but species in this
taxonomic group could not be identified in the field and all were
identified only as Cyanophyta.

b. Photographed Quadrats

A photographic census was also conducted to obtain additional
quantitative estimates of the jetty epibiota, and to provide a more
permanent record of biota at each level. Color photographs were obtained
of the same rock faces (i.e., seaward, landward, outer, inner, top) at
all station levels using a Nikonos III camera with flash attachment.



12



The camera was equipped with a 35-mm f2.5 Nikkor lens combined with a
35-mm closeup lens outfit and a rectangular quadrat frame (13 x 18.5 cm).
As noted for the line-transect census, all faces and levels were located
without reference to the attached biota.

Photographs were analyzed in the laboratory using a slide projector
and a screen with 50 computer-generated random points. One of 10
different screens was selected by random number for analysis of slides
from each level. Actual rock surface area examined in each slide was
100 cm^ for the north jetty stations and 150 cm^ for the south jetty
stations. Organisms occurring under the 50 points in each photograph
were identified, and percent cover estimates for each species, based on
the proportion of points occupied, were calculated for each level (i.e.,
250 points/level). Photographic analysis differed slightly from line-
transect analysis. Blue-green algae were not assessed in photographs
since they could not always be detected, even when present. Furthermore,
when there was uncertainty about whether biota existed under a point in
the photographs (due to shadows, poor picture quality, etc.), that
point was discarded and percent cover estimates were based on the number
of analyzable points only.

c. Suction Samples

Motile epifaunal invertebrates were sampled using a modified
underwater slurp gun. The levels sampled were +1 m, MLW, -1 m, and -2
m at all stations, except at the inner stations on each jetty, where
shallow depths precluded collection at the -2-m level. Three replicate
samples were obtained at all levels by placing the opening of the slurp
gun (4-cm diameter) flush against a rock face and vigorously pulling
the suction rod. Each replicate consisted of five suctions pooled from
different rock faces picked haphazardly. The gun was modified so that
suction was obtained by venturi action; incoming water through holes
drilled in the barrel was filtered through a 1-mm mesh screen. Contents
of the slurp gun were emptied into a gallon jug after each suction. To
prevent loss of organisms, the mouth of the jug was covered by a 1-mm
mesh screen having an opening just large enough to permit insertion of
the slurp gun barrel, and the jug was capped except when collections were
being added. After the five collections comprising each replicate had
been placed in the jug, the container contents were sieved through a
1-mm mesh screen and preserved in a 10% formalin seawater solution. Due
to some water leakage around the mouth of the gun and rock face during
the suction stroke, the exact surface area sampled per replicate was not
defined but approximated 65 cm2.

d. Fish Observations and Collections

Qualitative observations on ichthyofauna were made during
investigations of benthic flora and fauna on the jetty. Fish species
observed near the jetties by scuba divers were recorded, baited blackfish
traps were set at various locations on the jetty, and a beach seine was
pulled along the western side of the weir. In addition, fish species
were recorded from gill net collections made in conjunction with a related



13



investigation (Hales and Calder, 1979). Stomachs were removed from the
demersal species and preserved for laboratory analysis. In the laboratory,
the stomachs were washed in tap water and transferred to 50% isopropanol,
and contents of individual stomachs were sorted by taxa and counted. .
Colonial forms and fragments of animals were counted as one organism
unless abundance could be estimated by counting pairs of eyes (crustaceans) ,
otoliths (fishes), or other parts. Any food items (i.e., fish remains)
that might have been bait in blackfish traps were not included in the
analysis. Volume displacement of food items was measured using a graduated
cylinder, or estimated by using a O.l-cm^ grid (Windell, 1971).

4. Hydrographic Sampling

During every sampling period, surface and bottom water samples were
collected at all stations except SEI, which could not be reached by boat.
Samples were obtained using a Van Dorn bottle and the parameters measured
were temperature, salinity, dissolved oxygen, and water clarity. Water
temperature was measured from stem thermometers mounted inside the Van
Dorn bottles. Salinity was measured using a Beckman Model RS7B induction
salinometer, or a YSI Model 33 S-C-T meter. Dissolved oxygen was
measured using a YSI Model 51-B Dissolved Oxygen Meter, or by the modified
Winkler titration method (Strickland and Parsons, 1972). Water clarity
was measured using a Secchi disk.

5. Data Analysis

Community structure was evaluated through comparisons of species cover
or abundance, diversity indices, and cluster analysis. Where appropriate,
abundance estimates obtained from replicate sampling were statistically
compared using the non-parametric Mann-Whitney U test. Only the motile
macroinvertebrates were counted since most of the sessile fauna and flora
observed on the jetties were colonial.

Diversity indices used in the analysis of motile macroinvertebrates
included Shannon's index (H* ) and measurements of species richness (SR)
and evenness (J') as described by Margalef (1958) and Pielou (1975).
The expressions for these indices are as follows:



H* - - S P ± log 2 P ±
i=l

where s is the number of species and P^ is the proportion of the i
species in a collection,



th



SR = (s - 1)
log e n

where s is the number of species and n is the number of individuals
in a collection, and



J' =



H'
log 2 s



14



These measures were computed on data from pooled replicates of suction
samples at each level since pooling the replicates provided a larger
sample size and a more representative estimate of community diversity
at a site. Diversity of the sessile biota which generally could not be
counted was limited to comparisons of the number of species (s) observed
in photographs and along line transects.

Cluster analysis was used to determine patterns of similarity among
stations. The quantitative measure used in all analyses was the Bray-
Curtis coefficient (Boesch, 1977):



<ki



S jk 1 I (x. . + x k .)

where x^ and x, . are the number of individuals of the i species in

two collections under comparison. A normal analysis was completed on the
site groups using modified data sets and a flexible sorting strategy with
a standard 3 value of -0.25. Data sets represented pooled collections
from the different levels at a site (station), separated by seasons.
Additional modifications to the data sets included log transformation
and deletion of taxa which occurred in only one collection, as well as
deletion of those taxa of uncertain identity. These deletions were made
to simplify the data sets and because "rare" species usually do not have
definable distribution patterns, and can confuse interpretation of cluster
analysis.

Quantification techniques for food habits of fish are biased, depend-
ing on the method (Hynes, 1950; Pinkas et al. , 1971; Windell, 1971),
Therefore, the relative contribution of different food items to the
total diet was determined using three methods: percent frequency occurrence
(F) , percent numerical abundance (N) , and percent volume displacement
(V). From these, an index of relative importance (IRI) (Pinkas et al. ,
1971) was calculated for each prey species and higher taxon as follows:



IRI = (N + V) F

where N, V and F are the numerical, volumetric, and frequency percentages
as defined above. This index has proven useful in evaluating the
relative importance of different food items found in fish stomachs
(Pinkas et al. , 1971; McEachran et al. , 1976; Sedberry, 1983) and was
used in the present study to describe the food habits of each species.



15



IV. RESULTS AND DISCUSSION



1. Hydrographic Conditions

Water sample analysis for temperature, salinity and dissolved oxygen
(Tables 2 and 3) reflected expected hydrographic patterns for this area.
Temperature differences between surface and bottom waters were always
similar with a normal difference of less than 0.3°C. Lowest temperatures
(5.8° - 6.0°C) were observed during the winter and highest temperatures
(26.5° - 30.3°C) occurred during summer. Salinity measurements were
always high (34.5 - 36.1 °/oo) during the four-year study period since
Murrells Inlet receives no significant fresh water input. No salinity
data are presented for 1982 due to a faulty meter, but refractometer
estimates indicated that salinities were in the same range that year.
Dissolved oxygen values were generally high and near saturation values
since the shallow waters in this area are well mixed by wave action.
Finally, no consistent differences were noted between stations on the
north versus south jetty.

Water clarity varied considerably during the study, being mostly
dependent on tidal stage and wave action. Clarity increased during
flood tides and was often greatest on the exposed side of the north
jetty. The turbid waters from the inlet decreased water clarity at
channel (protected) stations on both jetties, especially during ebb tides.
The very shallow waters on the exposed side of the south jetty were also
generally more turbid than on the deeper exposed side of the north jetty.

2. Jetty Community Development

Data obtained from north and south jetty sampling indicate that a
diverse assemblage of biota colonized the rocks during the first four
years after construction. At least 25 species of algae, 195 species of
macroinvertebrates and 34 species of fish were observed or collected on
the jetties, with distinct temporal changes noted each year in the
community composition. Vertical gradients in the distribution of fauna
and flora on the rocks were also evident, particularly in the intertidal
zone. The following sections provide details* on the colonization,
community development, and distribution patterns observed on both jetties.

a. Sessile Biota .

Percent cover estimates for the sessile macroinvertebrates and
algal species are listed in Appendices A and B for the four north jetty
study sites, and Appendices C and D for the four south jetty sites.
Appendix E provides estimates of total biota cover on the rocks using the
two census techniques. The line-transect census (Appendices A and C)
generally provided more detailed information on community composition
at the different levels because taxonomic identifications were often more
refined than possible in the analysis of photographed quadrats (Appendices
B and D) . However, the latter technique did provide useful supplemental
information, particularly for the larger dominant biota which could be
easily identified.



16



Temperature, salinity, dissolved oxygen and water clarity measurements
collected during sampling periods at north jetty stations.



NEI
Surface Bottom



NEO
Surface Bottom



NPI
Surface Bottom



N70
Surface Bottom



Summer, 1979

Summer, 1980

Summer, 1981

Summer, 1982



28.4 28.3

28.4 28.2

26.7 26.7

28.2 28.0





TEMPERATURE


(°C)








28.4


28.4


28.4


28.3


28.3


28.3


28.3


28.2


28.9


28.8


28.8


28.8


26.5


26.6


26.6


26.7


26.6


26.6


28.2


28.2


28.2


28.2


28.2


28.2



Summer, 1979

Summer, 1980

Summer, 1981

Summer, 1982



35.6


35.5


35.5


35.5


35.7


35.7



35.5
35.5
35.6



SALINITY (°/oo)
35.5 35.5

35.5 35.4

35.6 35.8
NO DATA



35.5


35.5


35.5


35.4


35.4


34.5


35.8


35.8


35.8



DISSOLVED OXYGEN (mg/Q



Summer, 1979

Summer, 1980

Summer, 1981

Summer, 1982



6.9

6.7
6.8
5.0



7.0
6.2
6.4
5.0



7.0
6.1
6.9
5.1



6.9
6.0
6.7
4.8



6.4
6.7
6.5
4.8



6.7
6.6
6.6
4.6



6.9

7.0
6.7
4.7



6.8
6.8
6.8
4.8



Summer,


1979


1.9


Summer,


1980


0.7


Summer,


1981


2.5


Summer


,„ :2





WATER CLARITY (m)

2.3 1.5
0.8 1.0

2.4 1.6
NO DATA



1.8

1.0
1.6



17



Temperature, salinity, dissolved oxygen and water clarity measurements collected
during sampling periods at south jetty stations.



SEI

Surface Bottom



SEP
Surface Bottom



SPI
Surface Bottom



Surface Bottom



Spring, 1980

Summer, 1980

Fall, 1980

Winter, 1981

Summer, 1981

Summer, 1982



Spring, 1980

Summer, 1980

Fall, 1980

Winter, 1981

Summer, 1981

Summer, 1982



TEMPERATURE (°C)



24.7


24.1


24.7


24.0


24.2


24.1


29.3


29.3


30.2


30.1


30.2


30.3


15.7


15.7


15.0


14.9


15.2


15.2


6.0


6.0


5.9


5.9


5.8


5.8


26.9


27.0


27.4


27.3


26.9


26.9


28.2


28.2
SALINITY


28.5
(°/oo)


29.0


28.5


28.2


34.5


34.6


34.6


34.5


34.5


34.5


35.1


35.1


35.2


35.2


35.2


35.2


35.1


35.1


35.4


35.4


35.5


35.5


36.1


36.1


36.1


36.1


36.1


36.1


35.8


35.8


35.8


35.8


35.7


35.7



NO DATA



DISSOLVED OXYGEN (mg/lQ



Spring, 1980
Summer, 1980
Fall, 1980
Winter, 1981
Summer, 1981
Summer., 1982



7.9


7.9


7.5


7.6


8.0


7.9


6.9


7.0


7.1


7.5


6.9


6.8


7.8


7.9


8.2


8.3


8.2


8.3


10.2


10.4


10.1


10.3


10.4


10.1


6.8


b.4


6.6


6.4


6.9


6.9


5.2


5.2

WATER


5.2
CLARITY (m)


5.2


5.4


5.4



Spring, 1980
Summer, 1980
Fall, 1980
Winter, 1981
Summer, 1981
Summer, 1982



1.4
0.9
1.0
1.1
1.6



1.1
1.0
1.1
1.3
1.0



1.5
1.5
1.0
1.5
1.7



NO DATA



IS



(1) . Total Biota Cover and Number of Taxa

Estimates of total biota cover at the different levels of
north jetty stations indicated no consistent or marked differences
between inner and outer sites on the same side of the jetty (Appendix
E.l). Biota cover on the rocks one year after construction was generally
as great as in subsequent years (Fig. 2). This was primarily due to
early settling of blue-green algae and the barnacle Chthamalus fragilis
intertidally, and settling of the mussel Braahidontes exustus at lower
intertidal and subtidal levels.

Biota cover at inner and outer south jetty stations did differ
considerably in the spring of 1980, with outer sites having less cover
than inner sites at all levels where biota was present (Appendix E.2).
Rocks at the outer stations had only been submerged for 2-3 months as
compared to 7 months of submersion at the inner stations. By the summer
of 1980, biota cover at all levels on the rocks had increased to percen-
tages as great or greater than those found in subsequent sampling
periods (Fig. 3).

In the upper intertidal zone (1.5 m - 2.0 m above MLW) , biota cover
was often greater on the wave-exposed side as compared with the
sheltered side of the jetties (Figs. 2 and 3). This vertical extension
in the amount of biota cover on the exposed side is a common pattern
which has been observed in several other rocky intertidal systems (Lewis,
1972). Biota cover on the rocks of both jetties generally increased at
the lower levels, and differences between sides were not as great.
Because cover on the exposed side was rarely less than on the wave-
protected side (Figs. 2 and 3), it is unlikely that wave shock represents
a major source of mortality as noted in other rocky intertidal systems
(Dayton, 1971; Menge, 1978). However, wave energy in those systems is
often considerably greater than the moderate wave energy observed at
Murrells Inlet.

Comparisons of biota cover estimates obtained by line-transect versus
photographic census (Figs. 2 and 3) showed strong similarities except at the
highest intertidal levels. Blue-green algae were dominant in the upper inter-
tidal zone, and these species of algae were not assessed in the photographs.

Since many of the sessile organisms are colonial, species diversity
indices were not calculated on this component of the jetty communities.
However, an examination of the number of taxa found on the rocks indicates
that there were fewer species in the intertidal zone than in the subtidal
zone on both jetties (Figs. 4 and 5, Appendices A-D) . The more rigorous
physical environment associated with the intertidal habitat obviously
limits the number of species which can colonize this area as compared with
the less stressful subtidal environment.

In both the intertidal and subtidal zones, the number of taxa present
on the jetty within one year after construction was nearly equivalent to
or greater than the number found in later years (Figs. 4 and 5).
Additionally, there were no major or consistent differences in the number
of taxa found on the wave-exposed versus sheltered sides of the jetties.



19



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982



Figure 4. Total number of sessile taxa observed at intertidal and
subtidal levels of north jetty stations during line-
transect census.



22



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ntertida




• • Exposed Side

o- -o Protected Side



tr

Hi 30-
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JAN. JAN. JAN.

1980 1981 1982



Figure 5. Total number of sessile taxa observed at
intertidal and subtidal levels of south
jetty stations during line-transect census,



23



If wave stress at Murrells Inlet had been greater, differences in the
diversity of species might have been more apparent, as noted in other
rocky intertidal systems (Menge and Sutherland, 1976).

(2) . Community Composition

(a). North Jetty

Although estimates of total biota cover and the
number of taxa on the north jetty rocks did not change markedly over
the four-year study period, community composition of the sessile biota
did vary considerably between years (Tables 4 and 5; Figs. 6 and 7).
Major differences were also observed in the intertidal versus subtidal
community composition and, to a lesser extent, between the wave-exposed
and protected sides.

One year after jetty construction, the dominant intertidal species
at all stations were the barnacle Chthamalus fragilis and the mussel
Brachidontes exustus. These two species accounted for approximately
63% of all biota cover in this zone. The oyster Crassostrea virginica
and the barnacle Balanus eburneus were the only other fauna among the
ten dominant organisms found intertidally. Blue-green algae (Cyanophyta)
was the primary intertidal algal form during the first year. The
dominant species of blue-greens were Anacystis aeruginosa, Microcoleus
lyngbyaceous, and Calothrix Crustacea. These three species were noted
during all later sampling periods, as well. Other algal species found
on the rocks included the green algae Cladophora sp. (primarily
C. laetevirens) and Ulva sp., and the red algae Hypnea muscifovmis ,
Lomentavia baileyana and Herposiphonia tenella. Although barnacle and
mussel cover was generally similar on both sides of the jetty in 1979,
blue-green and green algae were the predominant algal forms on the exposed
side, whereas red algae were predominant on the protected side.

Chthamalus fragilis, Brachidontes exustus, Ulva sp. and Cyanophyta
continued to dominate the intertidal biota cover during the next three
years (Tables 4 and 5). In 1980, rock coverage by the different taxa
was similar to that observed in 1979 (Figs. 6 and 7), but by 1981, algal
cover had increased. The line-transect census indicated that blue-green
algae was more prevalent this year than in any other year. Larger
macrophyte coverage had also increased to a lesser extent, with green
algae {Viva sp., Enteromorpha sp., and Cladophora sp.) generally being
more common on both sides of the jetty than red algae (Gracilaria
foliifera, Porphyra sp., Hypnea musciformis and Polysiphonia sp.). By
1982, algal and mussel cover had decreased considerably. Barnacles
(C. fragilis) represented the dominant biota on the rocks, although blue-
green algae was also common.

The decline in algal and mussel cover during the last year of study
is not readily explained. Some of this decline may be attributed to
mortality. Additionally, both taxonomic groups were concentrated in
only the lowest portion of the intertidal zone during all four years,
and the MLW sampling level represented the upper limit for many of the



24



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Online LibraryRobert F Van DolahEcological effects of rubble weir jetty construction at Murrells Inlet, South Carolina → online text (page 2 of 7)