Seagrass: A Dynamic System

<p>Small herbivorous invertebrates consume algal epiphytes, reducing negative effects (e.g. shading) on seagrass. Much research to date has focused on crustacean grazers, and comparatively little on gastropods. The aim of this research was to 1) examine seasonal and spatial variation in seagrass and associated gastropods in a large, tidal estuary and 2) examine the response of seagrass and epiphytic algae to nutrient enrichment and grazing pressure in a laboratory experiment. Surveys were conducted in summer and winter of 2016 to assess the seasonal fluctuations in the associated gastropods and relationships with seagrass and epiphyte biomass within three sites in the Porirua Harbour. Seagrass, gastropods and epiphytic algae showed seasonal trends, including evidence of a loss of grazer control on epiphytes during winter. Potamopyrgus estuarinus, Notoacmea scapha, Diloma spp. and Micrelenchus spp. were the dominant gastropod grazers in the system in both seasons. The gastropod assemblage and seagrass characteristics differed between sites, likely in response to small scale differences in abiotic factors. Seagrass from Elsdon (a site with elevated nutrient levels) and Browns Bay (a relatively pristine site) were used to investigate the role of select grazers and nutrient enrichment on epiphyte and seagrass growth. Nutrient treatments represented nitrate and phosphate concentrations of Elsdon (High), a 20% increase (High+) and a control (no addition). Little evidence was found for epiphyte regulation by gastropods, nor did epiphyte loads increase with nutrient addition. Seagrass from Browns Bay responded more strongly to High+ treatments than that of Elsdon. The results suggest that seagrass from Elsdon is adapted to the site’s high nutrient loads, where seagrass from Browns Bay is not. The results of this thesis support prior research findings of high variation in seagrass over a small scale, and adds to the currently lacking information on the role of micro-grazers in New Zealand’s seagrass meadows.</p>

Seagrass: A Dynamic System

<p>Small herbivorous invertebrates consume algal epiphytes, reducing negative effects (e.g. shading) on seagrass. Much research to date has focused on crustacean grazers, and comparatively little on gastropods. The aim of this research was to 1) examine seasonal and spatial variation in seagrass and associated gastropods in a large, tidal estuary and 2) examine the response of seagrass and epiphytic algae to nutrient enrichment and grazing pressure in a laboratory experiment. Surveys were conducted in summer and winter of 2016 to assess the seasonal fluctuations in the associated gastropods and relationships with seagrass and epiphyte biomass within three sites in the Porirua Harbour. Seagrass, gastropods and epiphytic algae showed seasonal trends, including evidence of a loss of grazer control on epiphytes during winter. Potamopyrgus estuarinus, Notoacmea scapha, Diloma spp. and Micrelenchus spp. were the dominant gastropod grazers in the system in both seasons. The gastropod assemblage and seagrass characteristics differed between sites, likely in response to small scale differences in abiotic factors. Seagrass from Elsdon (a site with elevated nutrient levels) and Browns Bay (a relatively pristine site) were used to investigate the role of select grazers and nutrient enrichment on epiphyte and seagrass growth. Nutrient treatments represented nitrate and phosphate concentrations of Elsdon (High), a 20% increase (High+) and a control (no addition). Little evidence was found for epiphyte regulation by gastropods, nor did epiphyte loads increase with nutrient addition. Seagrass from Browns Bay responded more strongly to High+ treatments than that of Elsdon. The results suggest that seagrass from Elsdon is adapted to the site’s high nutrient loads, where seagrass from Browns Bay is not. The results of this thesis support prior research findings of high variation in seagrass over a small scale, and adds to the currently lacking information on the role of micro-grazers in New Zealand’s seagrass meadows.</p>

Grazing behavior and winter phytoplankton accumulation

Recent observations have shown that phytoplankton biomass increases in the North Atlantic during winter, even when the mixed layer is deepening and light is limited. Current theories suggest that this is due to a release from grazing pressure. Here we demonstrate that the often-used grazing models that are linear at low phytoplankton concentration do not allow for a wintertime increase in phytoplankton biomass. However, mathematical formulations of grazing as a function of phytoplankton concentration that are quadratic at low concentrations (or more generally decrease faster than linearly as phytoplankton concentration decreases) can reproduce the fall to spring transition in phytoplankton, including wintertime biomass accumulation. We illustrate this point with a minimal model for the annual cycle of North Atlantic phytoplankton designed to simulate phytoplankton concentration as observed by BioGeoChemical-Argo (BGC-Argo) floats in the North Atlantic. This analysis provides a mathematical framework for assessing hypotheses of phytoplankton bloom formation.

The dynamics and geographic disjunction of the kelp Eisenia arborea along the west coast of Canada

Eisenia arborea has a disjunct distribution along the west coast of North America. We detail the current distribution of E. arborea and use long-term records to examine how present-day shifts in E. arborea prevalence and abundance in British Columbia (BC), relative to the dominant stipitate kelp Pterygophora californica, may be driven by interactions between changing grazing pressure and warming water. We further speculate on how the disjunction of E. arborea arose. The ancestor of E. arborea likely dispersed from Japan to North America where glaciation disrupted its distribution and speciation occurred. As glaciers retreated E. arborea likely dispersed into BC from warmer waters in the south and/or expanded from refugia off Vancouver Island and Haida Gwaii. While E. arborea is uncommon, our records extend its range into Alaska and Washington State. Along western Vancouver Island, BC, under warming conditions, E. arborea prevalence and abundance increased where once-extirpated sea otters (Enhydra lutris) removed urchins. Where otters were absent, however, reduced summer wave heights, associated with warming, apparently allowed urchins to graze shallow-water kelps, which declined. We suggest that under warming conditions, sea otters may increase kelp resilience, with E. arborea becoming more prevalent in NE Pacific kelp forests.

Complementary Differences in Primary Production and Phenology among Vegetation Types Increase Ecosystem Resilience to Climate Change and Grazing Pressure in an Iconic Mediterranean Ecosystem

Plant primary production is a key factor in ecosystem dynamics. In environments with high climatic variability such as the Mediterranean region, plant primary production shows strong seasonal and inter-annual fluctuations, which both drive and interplay with herbivore grazing. Knowledge on the responses of different vegetation types to the variability in both rainfall and grazing pressure by wild and domestic ungulates is a necessary starting point for the sustainable management of these ecosystems. In this work we combine a 15 year series of remote sensing data on plant production (NDVI) with meteorological (daily precipitation data) and ungulate abundance (annual counts of four species of wild and domestic ungulates: red deer, fallow deer, cattle, and horses) in an iconic protected area (the Doñana National Park, SW Spain) to (i) estimate the impact of intra- and inter-annual variation in rainfall and herbivore pressure on primary production, for each of four main vegetation types; and (ii) evaluate the potential impact of different policy (i.e., herbivore management) strategies under expected climate change scenarios. Our results show that the production of different vegetation types differed strongly in their responses to phenology (a surrogate of the effect of climatology on vegetation development), water availability (rainfall accumulated until the phenological peak), and grazing pressure. Although the density of domestic ungulates shows a linear, negative effect on the primary production of three of the four vegetation types, differences in primary production and phenology among vegetation types increase ecosystem resilience to both climatological variability and grazing pressure. Such resilience may, however, be reduced under the conditions predicted by climate change models, if the moderate predicted reduction in rainfall levels combines with moderate to high densities of domestic ungulates, resulting in important reductions in primary production that may compromise plant regeneration, leading to irreversible degradation. New management strategies taking advantage of habitat heterogeneity and phenological alternation, more flexible stocking rates, and the redistribution of management units should be considered to mitigate these effects. The use of available remote sensing data and techniques in combination with statistical models represents a valuable tool for developing, monitoring, and refining such strategies.

Impacts of Marine Heatwaves on Algal Structure and Carbon Sequestration in Conjunction With Ocean Warming and Acidification

As the ocean warms, the frequency, duration, intensity, and range of marine heatwaves (MHWs) increase. MHWs are becoming a severe challenge for marine ecosystems. However, our understanding in regard to their impacts on algal structure and carbon sequestration is still deficient or fragmentary, particularly when combined with ocean warming and acidification. In this paper, we reviewed the impacts of MHWs individually and combined with ocean warming and acidification on regime shift in algal community and carbon sequestration of both macroalgae and microalgae. Solid evidence shows that MHWs cause the decline of large canopy macroalgae and increase of turf-forming macroalgae in abundance, leading to the regime shift from kelp forests to seaweed turfs. Furthermore, increased grazing pressure on kelps due to tropicalization facilitates the expansion of turfs that prevent the recovery of kelps through plundering light and space. Meanwhile, MHWs could trigger microalgal blooms and the intensity of algal blooms is regulated by the severity of MHWs and nutrient availability. MHWs could lead to the decrease of carbon burial and sequestration by canopy-forming macroalgae due to depressed growth and increased mortality. The effects of MHWs on the productivity of microalgae are latitude-dependent: negative effects at low and mid-latitudes whilst positive effects at high latitudes. Ocean warming and acidification may accelerate the shift from kelps to turfs and thus decrease the carbon sequestration by canopy-forming macroalgae further. We propose that MHWs combined with ocean warming and acidification would reduce the biodiversity and facilitate the thriving of morphologically simple, ephemeral and opportunistic turfs and diatoms in coastal oceans, and phytoplankton with smaller size in open oceans. This structure shift would not be in favor of long-term carbon sequestration. Future studies could be conducted to test this hypothesis and investigate the impacts of MHWs on carbon sequestration under future ocean conditions.

Factors influencing late-Holocene vegetation dynamics and biodiversity on Hallands Väderö, SW Sweden: A statistical evaluation

Forest composition characteristic of the Mid-Holocene has survived on Hallands Väderö, an island nature reserve off the south west coast of Sweden. Current veteran Tilia and Quercus trees contribute to a remarkably rich biodiversity of fungi, bryophytes, lichens and insects. Understanding which potential factors influence Holocene vegetation dynamics can support efforts to protect biodiversity, but the role of grazing and browsing has previously been difficult to evaluate because of the lack of long-term datasets. Palaeoecological analyses over the last c. 3000 years from a pond on the island reveal sustained presence of Quercus, Alnus, Tilia, Corylus and Ulmus, alongside increasing Fagus in recent centuries. Changes in grazing pressure have been documented since AD 1665 and a statistical approach was used to calculate the relative importance of grazing pressure, climate variability, and fire activity on the dynamics of selected taxa. Grazing was the main factor reducing population size of Fagus, Alnus, Tilia and Corylus on the island over the period AD 1665–1858, with warm winter temperatures and summer humidity having significant positive influences in the last millennium for Quercus, Alnus, Tilia and Corylus. The survival of large numbers of red-listed species is likely to be due to the continuity of large old trees, ancient forest composition and a distinctive disturbance history in a favourable climate.

Simultaneous effect of habitat remnancy, exotic species and anthropogenic disturbance on orchid diversity and abundance

Orchids are potentially useful as ecological indicators because of their sensitivity to habitat fragmentation and anthropogenic disturbance. While many studies explore the effect of single factors on orchid diversity, few investigate how the extent, configuration and condition of surrounding habitat affect whole orchid communities. Here, we unravel the effect of biological invasions, anthropogenic disturbance (i.e. grazing pressure, ecological condition), habitat fragmentation and climate on an Australian orchid community. We sampled 39 plots across nine sites in the Mount Lofty Ranges, Australia. We recorded the number of orchid species and number of individuals per species in mid-winter, early-spring and late-spring to account for the effect of season on species visibility, with 115 surveys in total. We ranked grazing intensity and ecological condition, and estimated cover of exotic species. We analysed the response of richness and diversity through generalised linear mixed models, and differences in species composition through non-metric multidimensional scaling. We also explored fruiting success in two species associated with floral resources in the surrounding habitat. Habitat configuration in the surrounding landscape had different effects at increasing radii. Patch-level orchid diversity was positively correlated with habitat edges in the immediate area, and with habitat cohesion at medium scales, whereas diversity was negatively correlated with increasing habitat area across larger surrounding areas. Orchids co-existed with exotic species but were negatively affected once exotic cover exceeded 20%. Species composition was correlated with both exotic cover and level of disturbance. Fruiting success was unrelated to floral resources of the surrounding vegetation, although associated with certain bee-attracting species. Our findings reveal a complex relationship between orchid communities and their surrounding environments suggesting that while orchids benefit from a somewhat disturbed landscape, they fail to thrive once exotic cover exceeds 20%. These idiosyncratic responses suggest orchid diversity may be unreliable as early-warning indicators of habitat disturbance.