Top-down and bottom-up factors in tidepool communities

Abstract Recent studies suggest that nutrient variation influences rocky intertidal community structure, however empirical evidence is rare. In the Gulf of Maine, tidepools that occur on seagull feeding roosts are potentially subjected to regular nutrient loading from seagull guano. The results of a survey conducted on Swan's Island, ME show that roost tidepools have very low macroinvertebrate and macroalgal diversity as well as very high phytoplankton biomass compared to non-roost tidepools. An experiment presented here tested basic food chain hypotheses in tidepool communities. These basic food chain models predict that in a tidepool with one trophic level (phytoplankton only), phytoplankton biomass will increase when nutrients are enriched. In contrast, these models predict that in two trophic level tidepools (phytoplankton and mussels) herbivory will prevent an increase in phytoplankton biomass when nutrients are enriched. A short term 2 Â 2 factorially designed field experiment was used to test this basic conceptual model using herbivory by mussels and enrichment with nitrogen as the main effects. The results of this investigation are consistent with the predictions of basic food chain models, and indicate that over the short time interval of a few days, herbivory by mussels is sufficient to maintain low phytoplankton levels following enrichment with nitrogen. Experimental enrichment with phosphorus in this study had no effect on phytoplankton biomass. The results of this study suggest that periodic pulses of nitrogen into tidepools will have little effect on phytoplankton biomass when mussels are present and that longer-term chronic nitrogen influxes may be driving the patterns of community structure in tidepools occurring on roosts.

Vital population rates across multiple spatial scales: A geostatistical analysis

Population and community level processes change across multiple spatial scales. Understanding these patterns and the factors that drive them are essential for the development of spatial approaches in fisheries management. Determining appropriately-resolved spatial scales is further complicated in temperate marine ecosystems by the generally high mobility and low site fidelity of temperate species compared to those in tropical reef systems. Spatial variability in per capita vital population rates (e.g., consumption, mortality, growth, survival) can directly affect the population level dynamics of managed species. Benthic habitat characteristics also change across spatial dimensions and may contribute to the differential spatial variability in vital rates. We used a set of bioenergetic equations to compute the daily per capita rate of consumption and mortality for individual groundfish species. These estimates used data from the US National Marine Fisheries Service Northeast Fisheries Science Center bottom trawls and food habits data sets. A geographic information system (GIS) was created to map these vital rates at multiple spatial scales. The spatial variability and spatial independence of vital rates were then explored using geostatistical techniques. Multivariate statistical approaches were also used to examine how habitat characteristics may be influencing spatial patterns in vital rates. Our results demonstrate the challenge of evaluating key population processes at the most appropriate spatial scales. Yet the demonstrated ability to integrate individual, population, and habitat information across multiple spatial scales should enhance our ability to manage our living marine resources. Keywords: spatial statistics, consumption, mortality, groundfish, fisheries management, essential fish habitat, NW Atlanti

Appendix C. Levene's test of heterogeneity of variances and figure of variability in recruitment, density, and cover.

Levene's test of heterogeneity of variances and figure of variability in recruitment, density, and cover

Appendix B. North–south differences in percent cover for barnacle cover.

North–south differences in percent cover for barnacle cover

Appendix D. Details of stepwise regressions and canonical correlation analyses.

Details of stepwise regressions and canonical correlation analyses

Appendix A. Details of the ANOVAs given in Table 1 and depicted in Fig. 1.

Details of the ANOVAs given in Table 1 and depicted in Fig. 1