Ecology of rocky subtidal habitats
Phase shifts between kelp beds and sea urchin barrens
The rocky subtidal ecosystem along the Atlantic coast of Nova Scotia alternates between two community states: kelp beds where herbivourous sea urchins (Strongylocentrotus droebachiensis) are rare and barrens where urchins dominate the macrobenthos. Cyclical alternations between these states have been driven by urchin population outbreaks and by sporadic introductions of an amoebic pathogen that causes mass mortality of urchins. For over 3 decades, research in the Benthic Ecology Lab has been directed at understanding the dynamics of this system by linking process to pattern through a range of descriptive and experimental approaches, in both field and laboratory settings. Important early successes were the isolation and identification of the pathogenic agent (a new species, Paramoeba invadens) and demonstration of the key role of temperature in mediating the range and severity of epizootics.
The mechanisms that initiate phase shifts to barrens in the shallow subtidal zone include: 1) onshore migration of deepwater urchins (with a thermal refuge from disease) resulting in the formation of grazing aggregations along the offshore margin of kelp beds; 2) increased growth and reproduction of urchins during these destructive grazing events, leading to increased larval production; and 3) increased settlement and recruitment of urchins in recently formed barrens. Based on these studies, we developed mathematical models that accurately predict rates of transition from the kelp bed to barrens phase, and identified biotic and abiotic factors governing these rates. We also used models of warming temperatures and increased storm severity to predict that the frequency of sea urchin disease outbreaks will increase, making the kelp bed state more resilient to sea urchin overgrazing. Understanding these relationships has important implications for assessing the overall health of the coastal ecosystem and the management of commercial species (eg, finfish, lobster, sea urchin) that use kelp beds as food, habitat, and nursery area.
Kelp bed resilience
Currently sea urchins are absent from kelp beds due to recurrent disease events. However, warming ocean temperatures, mesograzers, increased epiphytic encrustation and storm events are shifting the competitive interaction between kelp and turf-forming algae in favour of turfs. Replacement of kelp with turf-forming or invasive algae is characterized by massive losses of biogenic habitat and primary productivity. Using field and laboratory experiments we examined the direct and indirect effects of temperature on kelp growth and mortality. We also found evidence that initial kelp loss can concentrate mesograzers on remaining kelp fronds and accelerate impacts. Storm events can have a large defoliating effect on kelp beds, which is exacerbated by Membranipora encrustation. We predict that widespread kelp loss will cause concomitant reductions in food-web struture, trophic-level productivity, and export to surrounding systems. Detailed empirical data (collected over 4 decades at some of our sites) coupled with coastal surveys will provide a basis for examining spatial and temporal patterns of kelp bed health at multiple, ecologically relevant scales (metres to 100s of km).
We examined the production and fate of kelp detritus in the subtidal zone, and assessed its role in subsidizing deep-living sea urchins and small invertebrate populations. We used dietary tracers (fatty acids and stable isotopes) to track energy flow through grazing and detrital pathways. We documented the deposition and turnover of kelp detritus using a tow camera system. We found that detritus is an important food source for deep-living sea urchins (20-100 m depth) that can support a high reproductive rate. Our combined empirical and modeling approaches enable us to predict how climate change will impact detrital production along the coast.
Marine invasive species
We documented the establishment of two invasive species that have disrupted urchin-kelp dynamics along this coast. An epiphytic bryozoan Membranipora membranacea encrusts kelp fronds causing fragmentation and loss, and has repeatedly decimated kelp beds. By removing the dominant canopy species, M. membranacea has facilitated the invasion of a green alga, Codium fragile fragile, and its displacement of kelp in the shallow subtidal zone. In a series of laboratory and field experiments, we elucidated factors that may limit (or fail to limit) the establishment and growth of Codium in subtidal habitats. We showed that urchins avoid grazing Codium when other algae are available, and have lower rates of growth and reproduction when fed Codium compared to kelp. We identified an activated chemical defense (involving the breakdown products of DMSP) that may deter urchins and other herbivores from feeding on Codium. In field experiments, we confirmed that removal of the kelp canopy (mimicking the damaging effect of Membranipora) facilitates the establishment and growth of Codium, which in turn limits re-establishment of kelp. Codium is susceptible to dislodgement and fragmentation by wave action, which results in major losses during the stormy fall and winter months. However, the alga appears to adapt its morphology to the local hydrodynamic environment (becoming less bushy in wave-exposed locations) to reduce drag and risk of dislodgement.
Sea urchin grazing front
Invasive bryozoan on kelp
Drifting Codium fragile
Tow camera system
Tow camera system
Invasive bryozoan encrusting kelp