Leslie Hart
Spatial comparisons between two kelp forest communities differing in wave exposure
Abstract
Processes that structure communities can be determined by making spatial comparisons of ecosystems. In this study, we address the possibility that there are differences in community structure between sites and sampling days in two kelp forest communities that have differing wave exposures. The two sites that we studied were Hopkins Marine Station in Pacific Grove, California and Whalers Cove in Point Lobos State Reserve, California. Hopkins is known to have greater protection from wave exposure than Point Lobos (Graham et.al. 1997). To determine the difference exposure has on community structure between sites, we performed transects quantifying fishes, invertebrates, and algae on two days and performed statistical analyses on these data. We found that there is a site difference in species composition between Hopkins and Lobos; invertebrates and algae specifically have a strong site effect. There was no difference in species composition for sampling day; however, the differences in fish assemblages are affected by sampling day. Fish were also affected by the difference in the interaction of site and sampling day. The difference in swell at the two locations was the reason why invertebrates and algal assemblages had a site difference. Some species may not be able to withstand the harsh conditions that originate from higher wave exposure as other species. Fish had a strong sampling day effect and interaction effect between site and sampling day since they are mobile.
Introduction
Spatial comparisons of communities or ecosystems provide insights into the biotic and abiotic processes that structure communities. In the Aleutian Islands, kelp forests accessible to killer whales have been found to have higher urchin densities than protected kelp forests. The mechanism is thought to be killer whale predation on sea otters, a keystone species that regulates urchins (Estes et.al. 1998). Comparing Marine Protected Areas with areas that allow fishing in Tasmanian kelp beds has shown how the removal of lobsters, which regulate urchin populations, drive a decline in kelp abundance (Ling et.al. 2009). Kelp forest communities have been compared along the coast of California before and after El Nino in order to understand the scale at which various kelp forests were affected (Edwards 2004). Comparing an area in central California with thermal outfall from a power plant to a nearby, unaffected site gave rise to community-wide changes of species (Schiel et.al. 2004). In this study, we look for a difference in community structure of kelp forests in two locations that differ in swell.
To study the role wave exposure has on the community structure of kelp forests, we compared Hopkins Marine Station with Point Lobos. These locations differ in protection and wave exposure; Hopkins is more protected and less exposed to waves than Point Lobos (Graham et.al. 1997, Watanabe 1984). We conducted quantitative surveys to estimate abundances of fishes, invertebrates, and algae at Hopkins and Point Lobos on two different days. Sampling on two different days gave us a sense of the importance of daily variation.
Through this study, we set out to answer the following questions: Is there a difference in species composition between site and does it vary by taxonomic group? Is there a difference in species composition between sampling days and does it vary by taxonomic group? Do both site and sampling day affect species composition and does the interaction vary by taxonomic group? We found that a difference in invertebrates and algae assemblages differed in susceptibility to swell at the two sites.
Methods
To survey and quantify the difference in exposure between the two sites, we took transects of fishes, invertebrates, and algae on two days. Data was collected on two days to test if temporal variation might influence spatial comparisons.
The Study System
This quantitative study was conducted in October 2011 at Hopkins Marine Station, Pacific Grove, California, USA (36°37'13.69"N, 121°54'7.54"W) and at Whalers Cove in Point Lobos State Reserve (36°31'11.49"N, 121°56'18.32"W), which is a few miles south of Monterey and Carmel Bay, California, USA. Both of these locations are Marine Protected Areas. This ensured that fishing would not affect any differences in fish assemblages. At Hopkins, the substratum present consists of large granite blocks with sand patches inbetween (Lowry & Pearse 1973). The ocean conditions are typically calm with a slight surge considering that the area is fairly well protected from waves and surge (Lowry & Pearse 1973). The depth where we conducted research ranged from about 20-40 feet and the temperature was in the mid fifties. At Point Lobos, the kelp forest’s understory canopy was dominated by Pterogophora californica and Eisenia arborea. The substratum is composed of large boulders on a sandy bottom. This site has a greater exposure to swell than Hopkins (Graham et.al.1997). Data were collected at an average depth of 35 feet and the temperature was in the high forties.
The target species that were accounted for at each site consisted of 9 species of fish, 7 species of algae, and 13 species of invertebrates (Figure 1). These species were chosen as target species because they are present at one or both of the sites. Some of the species were rare and others were more abundant. Mobility, size, and abundance in species differed; these traits of species influenced estimates from one day to the next.
Is there a difference in species composition between Hopkins and Point Lobos?
In order to make comparisons about the species compositions between two sites, 14 buddy pairs were split into two groups: one group at Hopkins and the other at Point Lobos. The two sites were sampled for two days and the groups switched sampling the sites. At Hopkins, we performed a quantitative survey of three taxonomic groups (fishes, algae, and invertebrates) on the deep and shallow sides of the permanent cable in the reef. The permanent cable runs from east to west with eyebolts at every 10 meters. Each buddy pair was assigned a meter number that were each ten meters apart. Using SCUBA, the deep side (offshore) was examined first. One person reeled out the meter tape with a 90° heading from the cable and both researchers counted the target species abundances for 30 meters. We restricted data collection of the target species sighted to 1 meter from the meter tape on both sides of it, with the exception of fish. Since fish cannot be counted in a planar view like invertebrates and algae, they were counted per unit volume within 2 meters. This procedure was then repeated on the shallow side (onshore) of the black cable with a 270° heading. At Point Lobos, we performed the same quantitative survey as Hopkins but instead with two shallow side meter marks per buddy pair that were every 5 meters. Our meter marks originated from a temporary transect that was placed from north to south. 90° headings were taken off the temporary transect.
To determine a difference in species compositions of three taxonomic groups between sites, we created a multidimensional scaling plot of all target species. On this plot, points close together have similar species composition and points farther away do not have similar species composition. This will show the effect on site. We ran a PERMANOVA test to decipher the difference of species composition in relation to the site by looking at the P-value. In order to see which species had the largest affect on the difference in community composition between Hopkins and Point Lobos, we calculated the percent contribution of all species.
Does the difference in species composition between Hopkins and Point Lobos vary by taxonomic group?
The same transects performed at Hopkins and Point Lobos were used to determine if the difference in species composition between sites vary by fishes, invertebrates, and algae. We used multidimensional scaling graphs for fishes, invertebrates, and algae to see if site had an effect on species composition. We ran a PERMANOVA test for fishes, invertebrates, and algae to determine differences in species compositions; looking at the P-value showed whether there were differences or not. To determine which species had the largest affect on the difference in community composition between sites, we calculated the percent contribution of each taxonomic group’s species assemblage between Hopkins and Lobos. To discover how the source of variance of species played a role, we analyzed the variance percentage of fishes, invertebrates, and algae between the two sites.
Is there a difference in species composition between days?
To determine if there is a difference in species composition between days, we used the same transects. In order to test a difference in species composition between sampling days, we created a multidimensional scaling plot of all target species. This graph can depict an effect on sampling days. We ran a PERMANOVA test that can indicate the difference of species composition in relation to the sampling day through the P-value.
Does the difference in species composition between days vary by taxonomic group?
The same transects were used to determine if there was a difference in species composition between days for fishes, invertebrates, and algae. We created multidimensional scaling graphs for fishes, invertebrates, and algae in order to detect any differences in species composition between sampling days. We ran a PERMANOVA test to discover if fish, invertebrate, or algal assemblages differed in species compositions between days. The P-values that resulted from this test allowed us to determine any differences. We calculated the percent contribution of species to the difference in community composition found for fishes to determine which species had the largest affect to the differences between days. We analyzed the variance percentage of fishes, invertebrates, and algae between days to discover how the source of variance of species played a role.
Do both site and sampling day affect species composition?
To measure if both site and sampling day affect species composition, we used the same transects. In order to determine an interaction effect between site and sampling day, we obtained P-values derived from the PERMANOVA test.
Does the interaction between site and sampling day vary by taxonomic group?
We used the same transects to determine if the interaction between site and sampling day vary by fishes, invertebrates, and algae. We ran a PERMANOVA test to find a P-value that helped us determine if there was an interaction effect between site and sampling day for the three taxonomic groups.
Results
Is there a difference in species composition between Hopkins and Point Lobos?
We found that there is a strong effect of site on the species compositions for all species between Hopkins and Point Lobos (Figure 2, PERMANOVA: all species site effect, P=0.001). The species that mostly contributed to the strong site effects are Balanophyllia elegans (31.54%) and Pterogophora californica (12.34%).
Does the difference in species composition between Hopkins and Point Lobos vary by taxa?
Invertebrates and algal assemblages specifically have strong effects on site (Figures 3, 4, and 6, PERMANOVA: invertebrate and algae site effects, P=0.001).Site has an effect on invertebrate and algal assemblages not the other way around. The algae that contributed the most to the differences in site are Pterogophora californica (29.43%), Cystoseira osmundacea (22.02%), and Chondracanthus corymbifera (17.88%). The invertebrates that contributed the most to site effect are Balanophyllia elegans (53.27%) and Patiria miniata (16.82%). There was no site effect for fishes (Figure 5, PERMANOVA: fish site effect, P=0.319).
Is there a difference in species composition between days?
There is not a difference in species composition between days (Figure 2, PERMANOVA: sampling day effect, P=0.382).
Does the difference in species composition between days vary by taxa?
Fish are affected by sampling day (Figures 5 and 6, PERMANOVA: sampling day effect, P=0.043). Invertebrates and algae assemblages did not have a strong effect on sampling day (Figures 3, 4, and 6, PERMANOVA: sampling day effect, P=0.505, P=0.724). The fishes that made the highest contributions to sampling day differences are Sebastes atrovirens (32.68%), Embiotoca lateralis (18.16%), and Sebastes mystinus (13.6%).
Do both site and sampling day affect species composition?
The interaction effect between site and sampling day did not have an affect on the species assemblages (PERMANOVA: sampling day effect, P=0.373).
Does the interaction between site and sampling day vary by taxa?
Fish assemblages are affected by the interaction effect between site and sampling day (PERMANOVA: sampling day effect, P=0.015). Invertebrates and algae are affected by the interaction between site and sampling day (PERMANOVA: sampling day effect, P=0.926, P=0.569).
Discussion
The influence of wave exposure may be the determining factor for the site and sampling differences of invertebrate and algal communities between Hopkins and Point Lobos. The alga Pterogophora californica may have been more prevalent at Point Lobos than Hopkins since it consists of woody stipes that enable it to be more hydrodynamic (Figure 7). Since it is hydrodynamic, the chance of strong swells affecting the alga is minimal. Also, Pterogophora may not be affected by swell since it is only about 2.3 meters in height (Abbot & Hollenberg 1976). The energy from waves does diminish at greater depths. Balanophyllia elegans, orange cup coral, may have been more prevalent at Hopkins than Point Lobos because it is more protected at Hopkins (Figure 8). Wave exposure may be too harsh for this sessile species, and strong swell may even affect their recruitment.
Fish are not as susceptible to wave exposure as the invertebrates and algae. Invertebrate and algal assemblages are not likely to change between sampling days since algae is attached to substrate and many invertebrates have sedentary or sessile lifestyles. Fish had a difference in day effect since they are mobile and there was a larger surge on the second sampling day at both sites. During higher wave exposure, fish are able to take refuge, which makes it harder to perform a fish survey. On the first sampling day, more fish were spotted at both sites. The second sampling day consisted of large swell and high surge that led to less fish sightings at both locations. It would have been ideal to sample fish on two calm days. Fish also had an interaction affect between site and sampling day due to their mobility.
Algae were the only taxonomic group that had a sufficient number of transects performed (Figure 9). We may have needed more invertebrate transects, however our results were significant enough to show that there was a difference in species composition for site and not day. Fish need more transects considering the sampling size was not large enough (Figure 9). Fish may have also needed to be sampled for more than two days since they are mobile. Sampling algae and invertebrates for two days is sufficient enough to analyze species composition between sites and days since they are living on the substrate and do not have as wide of a range like fish.