Nitrogen loading has been linked to eutrophication and seagrass bed declines worldwide, yet early warning signs and potential mitigating factors are often less clear. Our objective was to use published nitrogen loading model results together with eelgrass habitat surveys from 7 bays in Atlantic Canada to assess linkages between nitrogen loading, tidal flushing and bivalve aquaculture on observed eutrophication indicators in eelgrass habitats. Field surveys revealed significant differences in primary indicators (annual algae, tissue nitrogen) and secondary changes in eelgrass bed structure, yet no large loss of eelgrass cover or biomass. Multivariate analyses found positive correlations between nitrogen loading and eutrophication indicators, with distinct clusters of high- and low-impact sites, and the mitigating effects of flushing time and aquaculture. Our results highlight that combining measures of nitrogen loading, eutrophication indicators and mitigating factors can help detect early warning signs and assess eutrophication risk to inform management and conservation of coastal ecosystems before significant losses of seagrass occurs.

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).

Eelgrass ( Zostera marina ) beds provide important habitat and food sources for a wide range of associated species both above- and belowground. Organic enrichment and nutrient loading from anthropogenic sources can change eelgrass canopy structure and macroinfauna community composition, making them important indicators of ecosystem health. In Atlantic Canada, there is growing concern about the impacts of finfish aquaculture on eelgrass habitats. To quantify these effects, we examined differences in environmental parameters, eelgrass bed structure and macroinfauna communities at increasing distances from a finfish farm in Port Mouton Bay, Nova Scotia and a reference site in an adjacent bay. We also compared the results to recently published large-scale survey results from the Atlantic coast. Results indicate increased organic enrichment and decreased eelgrass biomass, shoot density, and macroinfauna biomass closer to the farm. Moreover, community structure significantly differed between sites with some sensitive species disappearing while tolerant species increased closer to the farm. Changes in the macroinfauna community could be linked to observed differences in environmental and eelgrass bed variables. Our results provide new insights into the impacts of finfish aquaculture on eelgrass habitats in Atlantic Canada. We discuss possible assessment and monitoring metrics that would enable managers and regulators to evaluate the risk and potential changes to eelgrass habitat as a result of finfish aquaculture.

Seagrass beds and their associated species communities play key roles in coastal ecosystems. The importance of the ecological functions provided by eelgrass (Zostera marina) and macroinfauna are well understood; however, the spatial variation and linkage of the two are much less known. Here, we performed large-scale field surveys across three biogeographic regions in Atlantic Canada along the coasts of New Brunswick, Nova Scotia, and Newfoundland. First, we examined variation in eelgrass bed structure (shoot density, canopy height, biomass) and environmental parameters (tissue nitrogen and carbon, sediment organic content, microphytobenthos and annual algae) at 19 sites across the 3 regions. Next, we examined the variation in macroinfauna community composition and summary measures (species richness, diversity, total abundance, and biomass). We then linked the eelgrass bed structure and environmental variables to the macroinfauna community to determine what best explained observed patterns. Our results indicate that eelgrass bed structure and most environmental parameters varied at the site level, whereas most variation in the macroinfauna community was explained by region. Furthermore, the abundance of microphytobenthos was the best predictor of the macroinfauna community. We suggest that in moving forward with protecting and managing eelgrass habitats, eelgrass bed structure should be assessed on a site-by-site basis; however, benthic productivity (microphytobenthos) may be a useful tool in evaluating macroinfauna and ecosystem health on a region-scale.

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.

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.

Perennial seaweeds are dominant primary producers and foundation species along rocky shores, providing essential ecosystem functions and services. Although increasingly affected by various anthropogenic activities, the cumulative effects of multiple stressors are little known. We tested the interactive effects of nutrient enrichment and increased water temperatures on growth, nitrogen retention and carbon storage in juvenile Ascophyllum nodosum from Nova Scotia, Canada (44 degrees 29.9' N, 63 degrees 31.7' W) using a multi-factorial laboratory experiment. Temperature strongly affected growth, significantly reducing weight and length gain from 16 degrees C to 20 degrees C and 24 degrees C. Medium nutrient enrichment enhanced while high enrichment slowed rockweed growth at lower temperatures, yet these effects disappeared with warming. Nitrogen retention in rockweed tissue significantly increased with nutrient enrichment and decreased with warming, whereas carbon storage remained unaffected. These individual and interactive effects of nutrient loading and climate warming may alter the structure and function of rockweed habitats with potentially far-reaching ecological and economic consequences. (C) 2016 Elsevier B.V. All rights reserved.

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.

Ascophyllum nodosum (rockweed) is a dominant, habitat-forming seaweed on intertidal rocky shores in the North Atlantic and commercially harvested in Canada, Maine and Europe. Rockweed plant structure varies regionally, and several morphotypes have been identified in Atlantic Canada alone. Yet the regionality of canopy structure, associated species communities and the link between the two have not been well understood. Using large-scale field surveys and multivariate statistical approaches, we report distinct canopy structures between sites in southwest Nova Scotia and southwest New Brunswick, the two major rockweed harvesting areas in Atlantic Canada, as well as distinct associated community composition. We then demonstrate significant links between canopy and community structure. Importantly, plant and canopy structure, including length, circumference and density, were much better predictors of associated community structure than rockweed biomass, which is often used for single-species monitoring. Adding region or site as predictors further improved model fits. Therefore, measuring plant or canopy structure would strongly improve insight regarding ecosystem changes. Moreover, incorporating information about regional differences in canopy structure and the dependence of associated communities on those canopies is critical for spatially refined, ecosystem-based management of the rockweed harvest.

Rising temperatures are changing the distribution and abundance of species worldwide, yet the magnitude of warming varies regionally. Atlantic Canada lies in a zone of significant warming and harbors many cold-adapted seaweeds of ecological and economic importance. Using a factorial laboratory experiment, we tested the effects of increasing water temperature on the survival, growth, and nutrient content of rockweeds (Ascophyllum nodosum, Fucus vesiculosus), Irish moss (Chondrus crispus), kelp (Laminaria digitata), and the invasive Codium fragile ssp. tomentosoides from Nova Scotia (44°29.9′N, 63°31.7′W). In June 2014, species were exposed to typical spring–summer water temperatures (12, 16, 20 °C), a predicted increase in summer temperature (23 °C), and potential heat wave temperatures in shallow waters (26, 29 °C) for 9 weeks. Chondrus crispus and L. digitata experienced highest growth at 12 °C, F. vesiculosus and Codium at 16 °C, and A. nodosum at 20 °C. Survival was lowest in L. digitata with no survival above 20 °C, followed by rockweeds with low survival above 23 °C, while C. crispus and Codium exhibited high survival at all temperatures. There was some evidence for temporary acclimation and short-term survival at higher temperatures. Temperature stress did not affect carbon content but some species showed increased tissue nitrogen, potentially changing nutritional quality and the ability to store and cycle nutrients. These species-specific responses to increasing water temperature will result in shifts in species composition along Atlantic Canada’s rocky shore, altering seaweed canopies, their ecosystem structure and function, and the services they provide.

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.

Using large‐scale field surveys across 12 estuaries in two provinces in Atlantic Canada, we analyzed changes in phytoplankton and benthic macroalgal communities as well as the canopy structure of eelgrass beds and quantified their carbon and nitrogen storage with increasing eutrophication. As eutrophication increased, phytoplankton biomass increased on average 1.8 times and phaeopigments doubled. Among macroalgae, the epiphytic Ulothrix speciosa increased 40 times in New Brunswick, and benthic Ulva lactuca 670 times in Prince Edward Island covering 61% of the bottom. Eelgrass showed a significant increase in leaf length and declines in shoot density and aboveground and belowground biomass, consistent with increased shading by opportunistic algae. As eelgrass biomass declined, so did the carbon storage capacity of the habitat. Nitrogen storage only declined in belowground eelgrass beds due to increasing tissue nitrogen content above ground with eutrophication. Despite province‐ and species‐specific responses of primary producers to nutrient loading, principal component analysis revealed an overall shift from perennial eelgrass to opportunistic macroalgae and phytoplankton with eutrophication at the regional scale, indicating generalized eutrophication effects on primary producer assemblages.

Marine vegetated habitats provide essential functions and services to ocean ecosystems and human well-being. It is unclear, however, how different habitat types compare. Using large-scale field surveys, we compared the canopy and community structure between eelgrass and rockweed beds in Atlantic Canada and assessed their nitrogen retention, carbon storage, and habitat services. We then used binary network models of predator-prey interactions to determine food-web structure and its robustness to species loss. Despite disparate 3-dimensional canopy structure, both habitats significantly enhanced overall abundance and diversity of associated flora and fauna, including several commercially important species. Significant differences occurred in the species assemblages within and between habitats and were attributed to different settlement opportunities, food availability, predation risk, and maneuverability. While eelgrass plants had higher nitrogen content, rockweed canopies maintained 8-fold greater biomass and, thus, 14-fold greater nitrogen and 8-fold greater carbon retention per unit area. Both rockweed and eelgrass food webs showed similarities to other temperate and tropical seagrass webs, yet their robustness to the loss of most connected species including primary producers was among the lowest; underscoring their vulnerability to disturbances affecting the functionally dominant primary producers. The present study demonstrates that marine vegetation provides important habitat, nitrogen, and carbon storage services, yet the extent of these services depends on the foundation species and its architecture. Changes in canopy structure will therefore have profound effects on associated food webs and ecosystem services. Thus, as increasing human pressures on coastal ecosystems threaten the continued supply of essential functions and services, the protection of marine vegetated habitats should be a management priority.

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.

We examined the effects of native kelps, Laminaria longicruris de la Pylaie and L. digitata (Hudson) Lamouroux, and of the invasive alga, Codium fragile ssp. tomentosoides (Van Goor) Silva, on the composition and abundance of mobile benthic macrofauna and of turf algae by measuring the response of these assemblages to experimental removal of the respective macroalgal canopy. From June 2003 to November 2004, we censused macrofauna and measured canopy cover within 4 x 10 m strips of alternating Canopy Intact (control) and Canopy Removed treatments in both a Codium- and a Laminaria-dominated habitat in the rocky subtidal zone of a semi-protected embayment on the Atlantic coast of Nova Scotia, Canada. Macroalgal canopy cover fluctuated seasonally, peaking in September/October (69% cover in 2003; 55% cover in 2004) for Codium and in May 2004 (70% cover) for kelps, and with both canopy types reaching a winter minimum in January 2004 (22 and 28% cover, respectively). In both Codium and Laminaria habitats, significant effects of canopy removal on the overall macrofaunal assemblage were evident only during periods in which canopy cover in the Canopy Intact treatment was >= 50%. In the Codium habitat, 4 out of 11 characteristic taxa were more abundant in the Canopy Intact treatment, where taxonomic diversity also was higher. In contrast, 4 out of 11 characteristic taxa in the Laminaria habitat were more abundant in the Canopy Removed treatment and diversity was similar between treatments. Turf algae were sampled in November 2004 and, despite between-treatment differences in the light regime, there was no significant effect of canopy removal. Our results indicate that selection of algal habitats by mobile macrofauna is likely determined by the different shelter and foraging opportunities offered by these morphologically dissimilar ecosystem engineers. (c) 2006 Elsevier B.V All rights reserved.

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 m(2) 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.

: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.