Allison L. Schmidt

A comparison of epifauna and epiphytes on native kelps (Laminaria species) and an invasive alga(Codium fragile ssp tomentosoides) in Nova Scotia, Canada

Abstract We compared epifaunal and epiphytic assemblages on the invasive alga, Codium fragile ssp. tomentosoides, with those on native kelps, Laminaria longicruris and L. digitata, at a moderately exposed site on the Atlantic coast of Nova Scotia. Thalli of each algal host (Laminaria and/or Codium) were sampled in two stands with monospecific canopy cover and one mixed canopy stand (Laminaria and Codium) in June and November 2004 and February 2005. Epifaunal assemblages on both fronds and holdfasts differed between algal species and among months, but no differences were detected between mixed and monospecific stands for each host type. Fronds of Laminaria supported greater densities of gastropods and asteroids, while amphipods, harpacticoid copepods, and a specialist herbivore (Placida dendritica) were more abundant on fronds of Codium. Holdfasts of Codium supported greater densities of nematodes and bivalves. Diversity of epifauna was greater on fronds of Codium than Laminaria and similar on holdfasts of both species. Codium supported a greater density of epiphytes than Laminaria. Total frond area of Laminaria and/or Codium and density of epifauna per m2 of substratum did not differ between stands. Our results suggest that habitat selection by epibionts is likely determined by specific chemical, structural and morphological characteristics of the algal species, rather than the amount of habitable area available for colonization.

Food-Web Structure of Seagrass Communities across Different Spatial Scales and Human Impacts

Seagrass beds provide important habitat for a wide range of marine species but are threatened by multiple human impacts in coastal waters. Although seagrass communities have been well-studied in the field, a quantification of their food-web structure and functioning, and how these change across space and human impacts has been lacking. Motivated by extensive field surveys and literature information, we analyzed the structural features of food webs associated with Zostera marina across 16 study sites in 3 provinces in Atlantic Canada. Our goals were to (i) quantify differences in food-web structure across local and regional scales and human impacts, (ii) assess the robustness of seagrass webs to simulated species loss, and (iii) compare food-web structure in temperate Atlantic seagrass beds with those of other aquatic ecosystems. We constructed individual food webs for each study site and cumulative webs for each province and the entire region based on presence/absence of species, and calculated 16 structural properties for each web. Our results indicate that food-web structure was similar among low impact sites across regions. With increasing human impacts associated with eutrophication, however, food-web structure show evidence of degradation as indicated by fewer trophic groups, lower maximum trophic level of the highest top predator, fewer trophic links connecting top to basal species, higher fractions of herbivores and intermediate consumers, and higher number of prey per species. These structural changes translate into functional changes with impacted sites being less robust to simulated species loss. Temperate Atlantic seagrass webs are similar to a tropical seagrass web, yet differed from other aquatic webs, suggesting consistent food-web characteristics across seagrass ecosystems in different regions. Our study illustrates that food-web structure and functioning of seagrass habitats change with human impacts and that the spatial scale of food-web analysis is critical for determining results.

Effects of nutrient enrichment on seagrass population dynamics: evidence and synthesis from the biomass-density relationships

Summary The available data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment were analysed to assess the intraspecific mechanisms operating within seagrass populations and whether biomass–density relationships can provide relevant metrics for monitoring seagrasses. The response of shoot biomass and density to nutrient enrichment was dependent on the type of study; the short-term positive response of biomass and density in experimental studies reveals context-specific nutrient limitation of seagrasses. The long-term negative response of descriptive studies probably results from ecosystem-scale events related to nutrient enrichment such as increased turbidity, algal blooms, epiphyte loads and anoxia. Most seagrass species analysed lie in the nonthinning part of the theoretical biomass–density curves. A simultaneous increase in biomass and decrease in density, evidence of self-thinning, were only observed in 4 of 28 studies. The analysis of both the static and the dynamic biomass–density relationships revealed that the slopes increase under nutrient enrichment. Surprisingly, the species-specific slopes (log B-log D) were higher than one, revealing that the B/D ratio, that is, the average shoot biomass, increases with density in all seagrass species analysed. Nutrient enrichment further enhanced this effect as biomass–density slopes increased to even higher values. The main drivers behind the increasing biomass–density slopes under nutrient enrichment were the increase in shoot biomass at densities above a species-specific threshold and/or its decrease below that threshold. Synthesis. Contrasting short- and long-term responses of both biomass and density of seagrasses to nutrient enrichment suggest that the former, positive ones result from nutrient limitation, whereas the later, negative ones are mediated by whole ecosystem responses. In general, shoot biomass of seagrasses increases with density, and nutrient enrichment enhances this effect. Experimental testing of facilitation processes related to clonal integration in seagrasses needs to be done to reveal whether they determine the low incidence of self-thinning and the intriguing biomass–density relationships of seagrass species. The increasing slopes and decreasing intercepts of the species-specific dynamic biomass–density relationships of seagrasses and the decreasing coefficients of variation of both biomass and density constitute relevant, easy-to-collect metrics that may be used in environmental monitoring.

Population dynamics of an invasive green alga, Codium fragile subsp. tomentosoides, in tidepools on a rocky shore in Nova Scotia, Canada

ABSTRACT Since its introduction to Nova Scotia in the late 1980s, the invasive green alga Codium fragile subsp. tomentosoides has spread from rocky subtidal habitats to tidepools on the Atlantic coast. We monitored recruitment, growth, and survival of C. fragile, and potential biotic and abiotic factors that regulate these processes in three tidepools at different tidal heights on a wave-exposed rocky shore over 4 y (2000–2003). Large seasonal and interannual fluctuations in population density (up to 520 plants · 0.25 m−2) were driven by recruitment of small thalli (≤ 2 cm length) in summer and subsequent mortality of larger plants in fall and winter. Variation in the timing and magnitude of recruitment among years may reflect differences in the mode of reproduction, with intensive recruitment via dispersing propagules establishing the dense populations that in subsequent years produce new thalli vegetatively. Growth rates of new recruits increased with water temperature between June and September. Survival of marked plants steadily decreased during summer and fall (to ≤ 20% by November 2001) and was greater in recruits transplanted to deeper and more wave-sheltered microhabitats within pools. Two pools in the low intertidal zone had lower temperatures, greater water movement, and fewer grazers than a third pool high on the shore. These environmental differences may account for variation in growth and survival of C. fragile among pools at different intertidal heights.

Comparative analysis of different survey methods for monitoring fish assemblages in coastal habitats

Coastal ecosystems are among the most productive yet increasingly threatened marine ecosystems worldwide. Particularly vegetated habitats, such as eelgrass (Zostera marina) beds, play important roles in providing key spawning, nursery and foraging habitats for a wide range of fauna. To properly assess changes in coastal ecosystems and manage these critical habitats, it is essential to develop sound monitoring programs for foundation species and associated assemblages. Several survey methods exist, thus understanding how different methods perform is important for survey selection. We compared two common methods for surveying macrofaunal assemblages: beach seine netting and underwater visual census (UVC). We also tested whether assemblages in shallow nearshore habitats commonly sampled by beach seines are similar to those of nearby eelgrass beds often sampled by UVC. Among five estuaries along the Southern Gulf of St. Lawrence, Canada, our results suggest that the two survey methods yield comparable results for species richness, diversity and evenness, yet beach seines yield significantly higher abundance and different species composition. However, sampling nearshore assemblages does not represent those in eelgrass beds despite considerable overlap and close proximity. These results have important implications for how and where macrofaunal assemblages are monitored in coastal ecosystems. Ideally, multiple survey methods and locations should be combined to complement each other in assessing the entire assemblage and full range of changes in coastal ecosystems, thereby better informing coastal zone management.

Regional-Scale Differences in Eutrophication Effects on Eelgrass-Associated (Zostera marina) Macrofauna

Eutrophication has caused strong shifts from perennial seagrass to opportunistic macroalgae and phytoplankton in many coastal ecosystems worldwide, yet responses of the primary-producer assemblage can vary with regional environmental and nutrient-loading conditions. The wider consequences of this variable primary-producer response on the associated animal community are little known. We used large-scale field surveys across 12 study sites with low or high eutrophication levels in two geographic provinces in Atlantic Canada to examine region-specific responses of macrofauna associated with eelgrass beds. In both regions, abundances of all groups increased with eutrophication, but species richness of mobile fishes and invertebrates decreased. Generally, filter feeders, epibenthic detritivores and some herbivores increased, while more hypoxia sensitive species declined. Small fishes and invertebrate predators increased with eutrophication mirrored by decreases in their prey. Despite similar general trends, our results show distinct shifts in species composition in each geographic region associated with differences in food availability and predation refuge offered by phytoplankton and opportunistic epiphytic or benthic macroalgae as well as tolerance to an increasingly hostile physico-chemical environment. So far, the continued persistence of eelgrass beds at our “highly” eutrophied sites indicates intermediate eutrophication levels with short-term benefits for some species. However, the loss of sensitive species and decrease in species richness highlight that eutrophication has already changed seagrass ecosystems in Atlantic Canada. Our work suggests that mitigating these changes will require regional-scale management.

Large-Scale Differences in Community Structure and Ecosystem Services of Eelgrass (Zostera marina) Beds Across Three Regions in Eastern Canada

Eelgrass (Zostera marina) forms extensive beds in temperate coastal and estuarine environments worldwide and provides important ecosystem services, including habitat for a wide range of species as well as nutrient cycling and carbon storage. However, little is known about how eelgrass ecosystem structure and services differ naturally among regions. Using large-scale field surveys, we examined differences in eelgrass bed structure, carbon and nitrogen storage, community composition, and habitat services across three distinct regions in Eastern Canada. We focused on eelgrass beds with low anthropogenic impacts to compare natural differences. In addition, we analyzed the relationships of eelgrass bed structure with environmental conditions, and species composition with bed structure and environmental conditions, to elucidate potential drivers of observed differences. Our results indicate that regional differences in eelgrass bed structure were weakly correlated with water column properties, whereas differences in carbon and nitrogen storage were mainly driven by differences in eelgrass biomass. There were distinct regional differences in species composition and diversity, which were particularly linked to temperature, as well as eelgrass bed structure indicating differences in habitat provision. Our results highlight natural regional differences in ecosystem structure and services which could inform spatial management and conservation strategies for eelgrass beds.

Potential impacts of finfish aquaculture on eelgrass (Zostera marina) beds and possible monitoring metrics for management: a case study in Atlantic Canada

Eelgrass (Zostera marina) has been designated an Ecologically Significant Species in Atlantic Canada. The development and rapid expansion of netpen finfish aquaculture into sensitive coastal habitats has raised concerns about the impacts of finfish aquaculture on eelgrass habitats. To date, no studies have been done in Atlantic Canada to examine these impacts or to identify potential monitoring variables that would aid in the development of specific conservation and management objectives. As a first step in addressing this gap, we examined differences in environmental variables, eelgrass bed structure and macroinfauna communities at increasing distances from a finfish farm in Port Mouton Bay, a reference site in adjacent Port Joli Bay, and published survey results from other sites without finfish farms along the Atlantic Coast of Nova Scotia. Drawing on research done elsewhere and our results, we then identified possible metrics for assessing and monitoring local impacts of finfish aquaculture on eelgrass habitats. Our results suggest some nutrient and organic enrichment, higher epiphyte loads, lower eelgrass cover and biomass, and lower macroinfauna biomass closer to the farm. Moreover, community structure significantly differed between sites with some species increasing and others decreasing closer to the farm. Changes in the macroinfauna community could be linked to observed differences in environmental and eelgrass bed variables. These results provide new insights into the potential impacts of finfish aquaculture on eelgrass habitats in Atlantic Canada. We recommend a suite of measures for assessment and monitoring that take into account response time to disturbance and account for different levels of eelgrass organizational response (from physiological to community).