Intertidal community structure and oceanographic patterns around
Santa Cruz Island, California, USA.
Carol A. Blanchette, Bernardo R. Broitman and Steven D. Gaines

[pdf] Marine Biology (2006) 149:689-701

Poster presented at EPOC 2003

[url] Related article in Limnology & Oceanography


The Santa Barbara Channel Islands (Santa Cruz Island and nearby Santa Rosa Island) are located in an extremely dynamic oceanographic setting. Strong gradients in environmetal conditions are observed over relatively small spatial scales (i.e. 100 km). These gradients in environmental conditions are stable over time with the south shores of Santa Cruz Island (SCI) experiencing strong wave action and the influx of warmer subtropical waters. The north shores of SCI are relatively protected for wave action and experience colder water temperatures originating in the Point Conception upwelling center. The patchy environmetal setting creates a unique opportunity to test current theories of community regulation in rocky intertidal communities. A composite from a 6-year time series of sea surface temperature (SST) from the Advanced Very High Resolution Radiometer (AVHRR) satellite sensor evidences the stability of the thermal gradientacross the shores of SCI (fig. 1).

Figure 1. Mean SST from the 6-year AVHRR time series of weekly means. The nominal pixel resolution is 1 km

SST analyses
In order to characterize site-scale patterns of SST, we utilized the 6-year time series of SST from the Advanced Very High Resolution Radiometer (AVHRR) (Kenneth S. Casey and Peter Cornillon. A comparison of in situ-based sea surface temperature climatologies. Journal of Climate, Vol 12, No 6, pp. 1848-1863, 1999).
Correlation analyses
We studied the role of different processes structuring the intertidal community through Spearman rank correlation (rs) analyses (Zar, 1996). The influence of biophysical variability was examined through the correlation between the long-term mean SST and the abundance of different functional groups across all sites.

Spatial community patterns
Patterns if abundance of functional groups display a marked spatial pattern. Primary producers are more abundant in the western part of SCI while sessile filter-feeding invertebrates are more abundant on the east side of the island (fig. 2). A similar pattern is observed in the abundance of
Figure 2.Spatial pattern of abundance of primary space occupiers. Figure 3.Spatial patterns of abundance of invertebrate consumers.

Relationships to SST
Community patterns evidence strong bottom-up control. Invertebrate filter-feeders and macroalgae appear to respond directly to processes asociated to water temperature. Cold water sites exhibit higher abundances of algae while filter feeders show the opposite pattern. (Table 1)

Responses to temperature were observed across individual functional groups. The percentage cover of all sessile filter-feeders and primary producers strongly responded to variations in SST, except crustose algae. Bottom-up community pathways predict an increase in the abundance of primary producers, associated with increased availability of nutrients. Indeed, macroalgal functional groups and surfgrass are strongly associated to lower water temperatures. Sessile filter-feeders are related to SST in the opposite direction, with a marked increased in the cover of barnacles and mussels. The increase in cover of sessile filter-feeders may be physiological or demographic (see below).

Table 1. Temperature effects (rs) on the cover of different functional groups
(* = p < 0.1; ** = p < 0.05)
Groupmean AVHRR SST
Seastars 0.9959**
Herbivores 0.8117*
Barnacles 0.7247
Mussels 0.8407*
Macroalgae -0.8986*
Corticated algae-0.9856**
Crustose algae 0.5798
Ephemeral algae -0.8986*
Surfgrass -0.8117*

Sites with higher sea surface temperature (SST) exhibit higher rates of larval arrival (fig. 4). The relationship with SST is extremely high for both mussel and barnacle larvae. Adult abundances follow an equally consistent pattern with SST (Table 2). The adult abundance pattern that may be more related to recruitment limitation, as evidenced by the strong adult-recruit relationship (fig. 5).

Figure 4. SST vs. larval arrival.
Barnacles (Chthamalus spp. ,open circles) and mussels (Mytilus spp., black circles).

Additional support for a "bottom-up fueling" of community structure comes from growth rate data obtained at five sites in SCI. It can be seen that a strong relationship is observed with growth rate averaged across all years (fig. 5).

Figure 5. Temperature effects on the multi-year mean of growth of transplanted mussels. r = 0.6150

Community patterns
Across sites, no negative relationships were observed between the abundance of invertebrate consumers and cover of sessile invertebrates. Moreover, we observed significant positive correlations between the percent cover of barnacles and the densities of consumers (herbivores and carnivores). In the case of macroalgae, we found evidence of consumer pressure through strong negative correlations between total macroalgal cover, ephemeral and corticated algae with herbivores. Strong negative correlations among sessile filter-feeders and corticated,ephemeral and total macroalgae cover suggest competitive processes structuring the community. Some interesting relationships are observed which are in wide agreement with the patterns reported for Chile. Corticated algae have strong negative correlations with sessile invertebrates (mussels and barnacles). Mussels and barnacle adults are not negatively, but positively correlated across sites.

Table 2. Spearman rs between consumers and resources across the six study sites (* = p<0.1; ** = p<0.05)
Barnacles 0.6571 0.9429*....
Mussels 0.0857* 0.7143 0.7714...
Macroalgae -0.1429-0.7714-0.8286*-0.9429*..
Corticated algae-0.0857* -0.8286*-0.7714-0.8857*..
Crustose algae 0.4857 0.7714 0.5429 0.2000-0.5205.
Ephemeral algae 0.0857* -0.6571-0.6000-0.7143 0.8286*-0.4286
Surfgrass -0.6000 -1.0000**-0.9429**-0.7143 0.8286*-0.7714