Steven R. Dudgeon

Experimental evidence for the origin of alternative communities on rocky intertidal shores

Ecological theory of alternative stable communities suggests the switch between different species assemblages occurs when environmental conditions alter species composition so that it crosses a critical threshold and enters a different basin of attraction. The theory implies that once the threshold is crossed, rates of key ecological processes, such as predation and competition, will change. This conjecture was tested on sheltered intertidal shores of New England that are dominated by either mussel (Mytilus edulis) beds or algal (Ascophyllum nodosum) stands. Twelve sets of clearings of different sizes, which mimicked effects of ice scour, were created in algal stands, and mortality of transplanted mussels was monitored. Clearings were 1, 2, 4 and 8 m in diameter. Mussel mortality due to the predatory snail Nucella lapillus declined in a step-like manner in clearings > 4 m in diameter. Initial densities of the predator did not vary among experimental clearings or sites and were not correlated with mussel mortality. Mussel mortality was correlated with predatory snail densities during and at the end of the experiment. These results provide experimental evidence supporting conjectures about the switch between alternative states in ecological communities.

SCALE‐DEPENDENT RECRUITMENT AND DIVERGENCE OF INTERTIDAL COMMUNITIES

Sheltered rocky bays in the Gulf of Maine support communities usually dominated either by the fucoid rockweed Ascophyllum nodosum or the mussel Mytilus edulis. In order to test whether clearing size determines the subsequent pattern of recruitment by intertidal organisms, we examined four common species in clearings in stands of A. nodosum. Circular experimental clearings of 1, 2, 4, and 8 m in diameter, which mimicked severe ice scour, and uncleared controls were established at 12 sites spread across four bays. Recruitment of A. nodosum and of three species not common in A. nodosum stands (mussels, barnacles, and the rockweed Fucus vesiculosus) varied with clearing size and the patterns did not vary among bays. Recruitment of A. nodosum declined exponentially with clearing size. Fucus and barnacles recruited in greater densities to large clearings (≥4 m) compared to small clearings (<4 m). Recruitment of F. vesiculosus was also site specific, reflecting patterns of local abundance, but that of A. nodosum was not. Mussels recruited onto artificial substrates in fewer numbers in large clearings, but in marginally greater numbers to rock underneath these substrates in large clearings. Recruitment densities of mussels and barnacles were strongly site specific. These results suggest that (1) the rate of succession varies site specifically, and (2) the trajectory of succession, either convergence toward or divergence from the surrounding community, is contingent upon the interplay between species interactions and the size of the clearing.

Spatial distribution and reproductive phenology of sexual and asexual Mastocarpus papillatus (Rhodophyta)

Fierst J.L., Kübler J.E. and Dudgeon S.R. 2010. Spatial distribution and reproductive phenology of sexual and asexual Mastocarpus papillatus (Rhodophyta). Phycologia 49: 274–282. DOI: 10.2216/09-41.1 Species of the genus Mastocarpus exhibit two distinct life cycles, a sexual alternation of generations and an obligate, asexual direct life cycle that produces only female upright fronds. In the intertidal red alga, M. papillatus (Kützing) sexual fronds dominate southern populations and asexual fronds dominate northern populations along the northeast Pacific coast, a pattern of spatial separation called geographic parthenogenesis. Along the central coast of California, sexual and asexual variants occur in mixed populations, but it is not known whether they are spatially separated within the intertidal zone at a given site. We investigated reproductive phenologies and analyzed patterns of spatial distributions of sexual and asexual M. papillatus at three sites in this region. Sexual M. papillatus were aggregated lower on the shore at two sites and only reproduced during part of a year, while asexual M. papillatus occurred throughout the intertidal range at all sites and reproduced throughout the year. The distribution patterns of sexual and asexual M. papillatus are consistent with a hypothesis of shoreline topography influencing their dynamics of dispersal and colonization. Spatial and temporal partitioning may contribute to the long-term coexistence of sexual and asexual life histories in this, and other, species of Mastocarpus. The occurrence of geographic parthenogenesis at multiple spatial scales in M. papillatus provides an opportunity to gain insight into the phenomenon.

Variation in recruitment and the establishment of alternative community states.

Mussel beds and rockweed stands (fucoid algae) have been shown to be ilternative states on rocky intertidal shores in New England, and here the hypothesis that variation in recruitment provides opportunity for the development of alternative community states was tested. Disturbance by ice scour opens patches for development of alternative states, and in winter 1996-1997, 60 experimental clearings of differing sizes were established on Swans Island, Maine, USA. Half of the plots were re-cleared during the winter of 2010-2011. Recruitment data for barnacles, mussels, and fucoid algae collected from 1997 to 2012 were used to (1) test for persistence of scale-dependent thresholds, (2) estimate the magnitudes and sources of variation, (3) fit a surface of alternative states as defined by the cusp catastrophe, and (4) test if 1997 recruitment would predict 2010-2011 recruitment in re-scraped plots (i.e., a test of divergence, which is expected in systems with alternative states). For barnacles and mussels, recruitment varied enormously year to year and among sites, but showed consistent patterns over the long-term with respect to clearing size. Average recruitment prior to re-clearing was a good predictor of recruitment afterwards. In contrast, over 50% of the variance in fucoid recruitment was unexplained with weak effects among years and locations. Past fucoid recruitment was a poor predictor of subsequent recruitment. The cusp analysis indicated that fucoid recruitment defines the alternative states. Fucoid recruitment was largely unpredictable and suggests long-term, small-scale priority effects drive the development of alternative states. These observations strongly reinforce the notion that long-term and well- replicated experiments are necessary to develop robust tests of ecological theory.

Predicting Effects of Ocean Acidification and Warming on Algae Lacking Carbon Concentrating Mechanisms

Seaweeds that lack carbon-concentrating mechanisms are potentially inorganic carbon-limited under current air equilibrium conditions. To estimate effects of increased atmospheric carbon dioxide concentration and ocean acidification on photosynthetic rates, we modeled rates of photosynthesis in response to pCO2, temperature, and their interaction under limiting and saturating photon flux densities. We synthesized the available data for photosynthetic responses of red seaweeds lacking carbon-concentrating mechanisms to light and temperature. The model was parameterized with published data and known carbonate system dynamics. The model predicts that direction and magnitude of response to pCO2 and temperature, depend on photon flux density. At sub-saturating light intensities, photosynthetic rates are predicted to be low and respond positively to increasing pCO2, and negatively to increasing temperature. Consequently, pCO2 and temperature are predicted to interact antagonistically to influence photosynthetic rates at low PFD. The model predicts that pCO2 will have a much larger effect than temperature at sub-saturating light intensities. However, photosynthetic rates under low light will not increase proportionately as pCO2 in seawater continues to rise. In the range of light saturation (Ik), both CO2 and temperature have positive effects on photosynthetic rate and correspondingly strong predicted synergistic effects. At saturating light intensities, the response of photosynthetic rates to increasing pCO2 approaches linearity, but the model also predicts increased importance of thermal over pCO2 effects, with effects acting additively. Increasing boundary layer thickness decreased the effect of added pCO2 and, for very thick boundary layers, overwhelmed the effect of temperature on photosynthetic rates. The maximum photosynthetic rates of strictly CO2-using algae are low, so even large percentage increases in rates with climate change will not contribute much to changing primary production in the habitats where they commonly live.

Rocky Intertidal Zone

Rocky intertidal zone is defined by the tides and the presence of hard surfaces but the types of organisms, the number of species, and the distribution and abundance of individual species found in particular communities depend on the physical aspects of shores, the supply of resources, food and larvae from overlying water, the biological interactions among the species present, and the regional pool of species. These communities are supported by both algal primary production and attached suspension feeders such as barnacles and mussels that trap planktonic organisms and link the ocean’s productivity to the shore. Both invertebrate and vertebrate consumers are common and have significant impacts on community structure. While rocky intertidal shores often display a marked vertical zonation of fauna and flora associated with the rise and fall of the tides, various physical factors and biological factors can lead to complex patterns of distribution and abundance. The rocky intertidal zone has been an active area of research because the communities are often well defined and the common organisms, which tend to be small and slow moving or attached, are easily manipulated. Recently, there has been a growing interest in the role of larval supply and large-scale oceanographic conditions on community composition.