Total= 4+18+40+40+

Lit Cited= 6

[4] Assessing species assemblage composition in two California kelp forests: a quantitative comparison

Michael Hoban

Abstract: [18] To quantify the patterns of spatial variation in species composition within kelp forests, we assessed surveyed the abundances of fish, invertebrates, and algae at two sites in central California. [first state why this was done. What is the value of this?] We analyzed abundance data for patterns in variation between site, sampling day, and for possible interactions between site and day. We found significant differences in overall species composition between sites, both overall and for algae and invertebrates. A significant difference between sampling days as well as interaction between day and site likely masked differences in fish assemblage composition. These results indicate that species assemblage composition is variable within ecosystems at a local scale. Additionally, when estimating species composition in kelp forests, fish likely require multiple sampling days, whereas algae and invertebrates may be accurately assessed using adequate replication on a single day.

Introduction [40] [this was GREAT… but good to close with a couple of sentences summarizing what you found and concluded!]

Detailed knowledge of the composition of species assemblages contributes greatly to our understanding of ecological processes. Species abundance and distribution are critical baseline data for investigations of mechanisms that drive community structure and for testing of ecological theory (Nelson 1959). Assessment of community composition at individual sites provides a snapshot of local patterns, but despite similarity in overall habitat composition, heterogeneity may exist and assemblages (and their associated ecological interactions) may vary spatially (Brown et al. 1995). Multiple site assessments can be used to gain knowledge of spatial variation of abundance and ecological interaction and provide understanding of local differences between sites within similar ecosystems.

Giant kelp (Macrocystis pyrifera) forests constitute much of the shallow subtidal habitat to ~30m along the Pacific coast between central California, USA (37°06′N, 122°20′W) and Baja California Sur, Mexico (27°11′N, 114°23′W) (Edwards 2004). This diverse and productive ecosystem remains relatively constant in general habitat makeup, but assemblages and communities vary from site to site along its extent (Cowen et al. 1982, Foster 1990, Santelices 1990, Steneck et al. 2002, Edwards 2004). Here, we use quantitative methods to estimate community composition in two kelp forests on the central coast of California: Hopkins State Marine Reserve and Point Lobos State Marine Reserve. These sites differ markedly in swell and exposure regimes (Graham 1997) and as such, we hypothesize that there will be differences in community composition between them. We seek to understand how community structure varies between the two sites and which components of this variation are contributed by higher order taxonomic groups (fish, invertebrates, algae). Due to constraints on available space at Point Lobos, we sampled on two days. Observed community structure can vary with differing environmental conditions such as tide, swell, and turbidity (all of which varied between the days). Differences in condition between sampling days may affect divers’ ability to accurately assess community composition as well as the visibility, presence, or absence of certain species (particularly fishes). For this reason, we wish to expose any possible effect of sampling day on site differences (and relative contributions of taxa to that effect). We also look for combined effects of sampling day and survey site, also broken into taxa.

In examining these sources of variance we attempt to investigate the adequacy of our sampling design. We wish to understand whether the number of sampling days impacts our results, and whether this varies by taxa (i.e. if particular taxa are more or less vulnerable to sampling day variation). In addition to sampling days, we consider whether our replication (number of transects per site) was sufficient to accurately assess species abundance: both overall and among taxa. Finally, we discuss our relative ability to assess abundance of particular species and reasons for potential variation.

Quantitative assessment of multiple kelp forest sites allows comparison between different locations within an ecosystem. Understanding what variation exists within a habitat allows us to develop and test mechanisms that drive that variation. Community structure exists at multiple spatial scales and is organized according to multiple ecological principles and enhancing understanding at any individual scale increases understanding of the ecosystem as a whole.

Methods [40]

Study system and species

We quantified species abundances of 28 species (Table 1) using scuba at Hopkins State Marine Reserve (Hopkins) in Pacific Grove, California (36°37.2303’N, 127°54.1748’W) and Point Lobos State Marine Reserve (Lobos) in Carmel, California (36°31.2534’N, 121°56.3324’W) (Figure 1). We chose species that represent a wide range of size, mobility, and trophic classes. This allows us to make community level generalizations based upon those categories. Surveys were conducted by teams of divers at both locations on Oct. 11, 2011 and Oct. 13, 2011. Both sites are characterized by stands of Macrocystis kelp anchored on granitic rocky substrate with varying communities of understory algae (primarily articulated red coralline algae of the genera Bosiella and Calliarthron and various brown macroalgae, depending on site). Bottom relief at Lobos is commonly higher than at Hopkins (4-6m vs. 1-2m, respectively). Both sites have course granitic sand channels separating rocky reef outcroppings (Johansen and Austin 1970, Watanabe 1984). Survey depths for both sites were within similar ranges (Hopkins: 9.4m – 12.2m Lobos: 7.3m – 12.2m). [hey… like your Intro suggested.. key issue here is their different exposures to swell!!!! Seminal to study, so critical that you descrine that here. Also… although your subheading says “species” there is little here explaining the species we studied (Table X) and why we chose those species.]]

General approach [nice]

We counted individuals of 28 species of algae, invertebrates, and fish along 30m belt transects using scuba. Divers counted invertebrates and algae in a 2m wide path (1m on either side of the transect tape) and fish within a 2m cubic volume, encompassing 2m of benthos, 2m of vertical water column, and 2m in front. In the case of Macrocystis, we recorded stipe count per plant. Dive teams at Hopkins conducted two survey transects at a single location in reciprocal compass directions (90° and 270° magnetic) and at two separate locations in a single direction (90° magnetic) at Lobos. We used replicates of 30 transects at each sampling site. We swam transects at Hopkins at incremental meter marks along a permanent cable running N-S. We swam transects at Lobos at meter marks along a baseline laid on the reef at the beginning of each sampling day.

Does species composition differ between Lobos and Hopkins?

To test the null hypothesis that there was no difference in overall species composition between Hopkins and Lobos, we used a three-factor univariate variation of the multivariate permutational analysis of variance (PERMANOVA) focusing on the site effect component of the analysis. [excellent!] The dependent variable was overall species abundance. Site, sampling day, and site-day interaction were used as factors. [hey… what about the MDS and how to interpret those figures?]

Do fish, vertebrate, or algal abundances differ between Lobos and Hopkins?

To test the null hypothesis that species abundance between Hopkins and Lobos does not differ for each taxon surveyed (fish, invertebrates, algae), we used PERMANOVA to analyze variance in species abundance using the fish, invertebrate, or algal abundance as the dependent variable and focused on the site effect component. [excellent!!] For these hypotheses, we focused on the site effect component of the analysis.

Does species composition differ between sampling days?

To test the null hypothesis that there was no difference in overall species abundance between sampling days (day 1 or day 2, 10/11 and 10/13 respectively), we analyzed transect abundance data using PERMANOVA with overall species abundance as the dependent variable and sampling day as the primary factor.

Do fish, invertebrate, or algal abundances differ between sampling days?

To test the null hypothesis that there was no difference in species abundance for each surveyed taxon between sampling days, we used PERMANOVA to analyze taxon-level variance in species abundance using fish, invertebrate, and algal abundance as the dependent variables and sampling day as the primary factor.

Is there an interaction effect of survey site and sampling day on species composition?

To test the null hypothesis that there was no combined effect of sampling day and site on overall species abundance, we analyzed abundance data using PERMANOVA with overall species abundance as the dependent variable and sampling day and survey site as factors, examining the interaction effect.

Is there a day-site interaction effect on fish, invertebrate, or algal abundance?

To test the null hypothesis that there was no combined day-site effect on abundance among taxa, we used PERMANOVA to analyze taxon-level variance using the fish, invertebrate, or algal abundance as the dependent variable with sampling day and survey site as factors, examining the interaction effect. To further assess the adequacy of our sampling methods, we calculated components of variance for day and site and performed a post-hoc power analysis (N=32) relating number of transects to statistical power.

Results

Species composition variation between Lobos and Hopkins

Overall species composition differed significantly between Lobos and Hopkins (PERMANOVA, p = 0.001). A multidimensional scaling (MDS) plot shows distinct grouping of transects by site (Figure 2). Mean abundances per transect of higher level taxa at Lobos were (mean ±SD) fish: 0.09 ±0.11, invertebrates: 1.59 ± 3.07, and algae: 1.99 ± 2.82. At Hopkins, the three most abundant species (fish, algae, and invertebrates) were Sebastes atrovirens (0.300 transect-1), Cystoseira osmundacea (7.725 transect-1), and B. elegans (28.602 transect-1) (Table 2). At Lobos, the three most abundant species by taxa (fish, algae, and invertebrates respectively) were Oxylebius pictus (mean: 0.243 transect-1), Pterygophora californica (8.306 transect-1), and Balanophyllia elegans (9.692 transect-1) (Table 3). B. elegans and P. californica contributed most to the site-level difference (31.54% and 12.34%, respectively). There was a significant difference in both the mean number of Macrocystis stipes per transect (ANOVA, p<0.0001) and mean Macrocystis stipes per plant (ANOVA, p<0.0001) at Hopkins vs. Lobos (Figure 3). Hopkins had significantly more and denser Macrocystis plants than Lobos.

Species composition variation between Lobos and Hopkins by taxa

Species composition differed significantly between Lobos and Hopkins for algae and invertebrates (PERMANOVA, p = 0.001 for both) but not for fish (PERMANOVA, p = 0.319). Among algae, P. californica contributed most to site-level difference (29.43%), whereas B. elegans contributed most among invertebrates (53.27%) (Figure 4, Figure 5, Figure 6).

Species composition variation between sampling days

Overall species composition did not differ significantly between sampling days for all taxa (PERMANOVA, p=0.382).

Taxon composition variation between sampling days

Composition did not differ significantly for algae or invertebrates (p=0.724 and p=0.505 respectively) but was significantly different for fishes (p=0.043). S. atrovirens contributed most (32.68%) to fish composition differences between sampling days.

Day-site interaction effects on species composition

There was no significant interaction effect of sampling day and site on overall composition (PERMANOVA, p=0.375).

Day-site interaction effects on taxon composition

Site and day showed a significant interaction effect between sampling day and survey site on fish composition (PERMANOVA, p=0.015). There was no significant effect for algae (p=0.926) or invertebrates (p=0.569).

Discussion

There were significant differences in overall kelp forest species composition between Hopkins and Point Lobos. Invertebrate and algae composition also differed significantly between the two sites. Of surveyed taxa, only fish showed a significant effect of sampling day and of day-site interaction. These results support the claim that despite the wide geographic extent and relative overall similarity in habitat, there is spatial variability within kelp forests at the local scale. Hopkins and Lobos are similar kelp forest ecosystems, but various factors may affect local differences in species assemblages. Hopkins lies within the southern part of Monterey Bay and is protected from prevailing Pacific Ocean swell whereas Lobos is exposed. The two sites experience differing oceanographic conditions based upon their locations and may be subject to different regimes of larval supply, current, upwelling, and other processes that can affect supply and recruitment of organisms to the benthos as well as influence community makeup and available habitat. Lobos has markedly higher bottom relief whereas Hopkins has more sand flat, both conditions that may lead to differential settlement of invertebrates and algae (Foster 1975, Marc 1992, Schroeter et al. 1996).

In contrast to algae and invertebrates, fish composition did not differ significantly between sites, but did differ between days and showed significant day-site interaction effects. Many temperate reef fishes have been shown to differentially favor substrate type and algal cover (Holbrook and Schmitt 1989, Carr 1991). Thus, due to the variation in substrate types and differences in understory algal composition (particularly Pterygophora californica), we predicted that fish communities would vary between Lobos and Hopkins. Our results do not necessarily contradict this prediction. Rather, they bring to light potential issues with our sampling design as applied to surveying fish.

Our data show significant interaction effects on fish of survey site and sampling day. This indicates that between-day differences in our data potentially mask actual predicted between-site differences. The two days on which we surveyed differed markedly in conditions: swell and surge were higher on day 2, while visibility was considerably lower. This affects both fish behavior (e.g. refuge seeking versus open foraging) as well as divers’ ability to see and count them. These factors are considerably less likely to affect invertebrate (many are sessile, semi-attached, or relatively unaffected by water movement) or algae counts (all are attached). Dive teams surveyed different locations on different days, thus divers did not have the opportunity to reassess the same locations and take advantage of increased site familiarity. Due to interaction effects on fish, we suggest adding both transects and sampling days to adequately estimate assemblages. A variance component analysis shows that day contributes nearly 4 times the abundance variance as site for fish (versus 0% for algae and invertebrates), suggesting the need to increase the number of sampling days by at least 4 times to compensate (Figure 7).

A post-hoc statistical power analysis indicates that our replication (transects) was likely sufficient for algae, but potentially inadequate for both fish and invertebrates (Figure 8). As neither the fish curve nor that for algae comes to asymptote in our analysis, we have insufficient data to predict an adequate number of replicates, but clearly more will be beneficial. Despite an indication that more replication overall would further increase statistical power, certain species seem to have been adequately sampled, especially Embiotoca lateralis, Sebastes atrovirens, and Patiria miniata (Figure 9, Figure 10). Certain algal species may also benefit from increased replication, particularly Chondracanthus corymbifera and Dictyoneuropsis reticulata (Figure 11).