Macroalgal Mats Research Articles

A persistent green macroalgal mat shifts ecological functioning and composition of associated species on an Eastern Tropical Pacific coral reef

Global evidence of phase shifts to alternate community types is of particular concern because these new communities can provide fundamentally different and often novel ecosystem functions and services compared to the original community. Shifts of a diverse range of marine communities to dominance by green macroalgal mats have occurred worldwide, making it critical to understand their emerging functions and roles. We observed a green algal mat on two reefs in the Eastern Tropical Pacific, with one persisting for >10 years on a reef with stable herbivore populations and no known sources of anthropogenic nutrients. These mats supported a more speciose macroalgal community with fewer taxa present in the adjacent coral community and facilitated growth of an associated understory macroalgal species by reducing herbivory pressure and possibly enhancing nutrient supplies within the mat community state. These results demonstrate a weakening in the processes controlling reef community structure as a result of the shift in composition associated with the macroalgal mat, creating a positive feedback supporting mat persistence. These novel ecosystem functions generated by this alternate community state illustrate the importance of further research on community shifts, which will become increasingly common in the Anthropocene.

Foraminiferal Distribution Pattern and its Ecological Implications in the Koswari and Van Islands of the Tuticorin Group of Islands

Objectives: The present investigation attempts to map the foraminiferal distribution pattern, its ecological significance, and the elemental composition of the Koswari and Van islands. Methods: Foraminiferal distribution patterns (both planktic and benthic) were recorded, and textural and geochemical analyses were carried out in the present study. Findings: The coastal environments are generally highly dynamic regions of the earth that possess imprints of global climatic variations. The islands of the Gulf of Mannar emerged during the last interglacial period. These islands are getting eroded due to the long-term impact of coral mining and climatic changes. The excess removal of corals from Koswari Island adversely affected the live coral cover of the Island. The Van island is in proximity to the Tuticorin coastal city and one-fourth of the Island has already become submerged due to several natural and anthropogenic factors. Both the islands are affected by the world coral bleaching event witnessed from 2014-2017. This study is an attempt to record how the foraminiferal assemblages of Koswari and Van islands responded to the world coral bleaching event. In the present investigation, we identified fifteen benthic foraminiferal species and two planktic foraminiferal species from both islands. The foraminiferal assemblages mark the abundance of Neorotalia calcar, Quinqueloculina seminulum, Elphidium crispum, Amphistegina lessonii, and Rotalidium annectens. The main ecological parameters which govern the foraminiferal distribution in these islands are coral reef flats, carbonate environments, and shallow coastal waters. Neorotalia calcar and Quinqueloculina seminulum are recorded as the most abundant species of both Koswari and Van islands. N. calcar prefers macroalgal mat as substratum,while the frequent occurrence of deformed tests and stress-tolerant species such as Q. seminulum indicate the influence of pollution in the coastal area. These recorded species are considered characteristic reef benthic foraminifers which prefer coarse sandy substratum. The other abundant species are Elphidium crispum, Amphistegina lessonii, and Rotalidium annectens, which are widely distributed in the inner shelf sediments of Tamil Nadu and Kerala coasts. Textural attributes of the Koswari and Van islands reflect its high energy environment, and the geochemical signature shows the high calcareous content with typical coral reef foraminifera associated with such sediment nature. Novelty: The study tried to map the effect of the global coral bleaching event of 2016 on Koswari and Van islands of the Tuticorin group of Islands in the Gulf of Mannar using foraminifera as a proxy. Keywords: Foraminifera; Coral reefs; Ecology; Coral bleaching; Gulf of Mannar

Effect of floating and attached macroalgal mats on the decomposition of riparian leaf litter

In freshwater lakes, macroalgal mats may easily float on the water surface or attach on the hard substrates. However, the number of studies assessing the effect of this mat on leaf litter decomposition is scarce. Further, the role of aquatic invertebrates in this process remains unexplored. To investigate the impact of macroalgal mat (Cladophora glomerata) on the decomposition of leaf litter, leaves of Neosinocalamus affinis (NA) and Ficus virens (FV) were placed in the litterbags (5 mm mesh in diameter), incubated them at the water surface (Up, U) and sediment surface (Down, D) with or without mat for 65 days in Jianhu Lake in the Three Gorges Reservoir area, China, in July 2020. We found that the average decomposition rate at the corresponding locations was always higher with mat than without mat. Under the same conditions, the decomposition rate of FV was significantly higher than that of NA. The average abundance of invertebrates was higher without mat than with mat. The abundance of invertebrates was higher at the with mat habitat and D location than at the without mat habitat and U location. The macroalgal mat had no significant effect on the microalgal abundance and microbial respiration rate, but the locations had a significant effect on them. These results indicate that the macroalgal mats may release dissolved organic matter, which can be directly utilized by microbes to promote decomposition. Our results highlight the need for more sophisticated studies to discern macroalgal potential impact on ecosystem carbon flow and nutrient cycling pathways.

Effects of Invasive Seaweed Depend on Underlying Physical Habitat Structure

Photo 1. A Green heron, Butorides virescens, forages on a mudflat covered with macroalgae. The dominant alga, Agarophyton vermiculophyllum, is visible as tufts of red-brown thalli and intermixed with Ulva spp., including Ulva lactuca. Photograph credit: Alice Besterman. Photo 2. Agarophyton vermiculophyllum forms mats up to 5 cm in depth above mudflat sediments, and generally between 2 and 3 cm deep. Here, a mat of Agarophyton has accumulated at the edge of a salt marsh, with some stems of Spartina alterniflora visible. The eastern mud snail, Tritia obsoleta (formerly Ilyanassa obsoleta), uses Agarophyton as refuge and substrate for egg-capsule deposition. Photograph credit: Alice Besterman. Photo 3. We used spotting scopes to observe shorebirds foraging across expansive mudflats exposed at low tide in the Virginia Coast Reserve LTER. We identified birds to the lowest taxonomic level, as well as the substrate they were used for foraging (bare mud, macroalgal mats, shallow water, oyster reefs, etc.). In this photograph, oyster (Crassostrea virginica) reef patches are visible in the foreground of the mudflat. Photograph credit: Alice Besterman. Photo 4. Hundreds of shorebirds including Limnodromus griseus (Short-billed dowitcher), Pluvialis squatarola (Black-bellied plover), and Calidris alpina (Dunlin) forage on a bare, topographically smooth mudflat in the Virginia Coast Reserve. Photograph credit: Alice Besterman. These photographs illustrate the article “Mudflat geomorphology determines invasive macroalgal effect on invertebrate prey and shorebird predators” by Alice F. Besterman and Michael L. Pace published in Ecology. https://doi.org/10.1002/ecy.3540.

Assessment of the causes of Hg bioaccumulation in the fish of a Mediterranean lagoon subject to environmental management interventions

In a Mediterranean lagoon characterized by high levels of Hg in the sediment, Hg content in commercial fish species was determined, and, in order to establish Hg concentration in the water column, Diffusive Gradient in Thin films technique (DGT) was used. The results allowed to state (1) the disturbance of the surface sediment and its resuspension in the water column did not cause detectable increase in Hg releases, (2) the East basin acted more than the West one in contamination of fish species, (3) the small-sized sea-breams (<400 g) were more contaminated than larger sizes ones (>400 g). We concluded: (1) fish contamination may also depend on direct contact with releases of the contaminant and small, gregarious and less sedentary subjects are more likely to incur releases of Hg; (2) Hg releases can be greater in areas with a relatively low sedimentary organic matter load compared to areas subjected to high density macroalgal mat, regardless of the amount of Hg present in the sediments; (3) wind transport of plant masses in low energy storage areas, may constitute an increase factor of Hg in the sediment. A critical review of the results was made in comparison with the available literature reports and some hypotheses were proposed regarding the possible dynamics and availability of the contaminant.

Seagrass meadows provide multiple benefits to adjacent coral reefs through various microhabitat functions

ABSTRACT Tropical seagrass meadows and coral reefs often function as interconnected marine habitats, but they are often studied and managed as homogenous units. As macrohabitats, seagrass meadows provide important benefits to adjacent reef ecosystems by acting as natural filters of sediments and nutrients, and by providing critical feeding, nursery, and refuge habitats for reef fishes and other fauna. Whilst the macrohabitat functions of seagrass meadows have been often acknowledged, their microhabitats functions have largely been neglected. The purpose of the study is to explore how seagrass meadows provide multiple benefits to adjacent coral reefs through various microhabitat functions. The paper reveals some of the diversity of microhabitats that seagrass meadows contain, such as macroalgal mats, rubble cavities, sand patches with sparse seagrass, anemone gardens, hard substratum, and sponges mixed with seagrass. We highlight the ways in which reef creatures have diversified and specialized in using these different microhabitats, and postulate that seagrass microhabitat diversity enhances the habitat function and faunal diversity of seagrass meadows.

An invasive macroalga alters ecosystem metabolism and hydrodynamics on a tidal flat

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 628:1-16 (2019) - DOI: https://doi.org/10.3354/meps13143 FEATURE ARTICLE An invasive macroalga alters ecosystem metabolism and hydrodynamics on a tidal flat Martin P. Volaric*, Peter Berg, Matthew A. Reidenbach Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904, USA *Corresponding author: mpv3a@virginia.edu ABSTRACT: Invasive macroalgae represent a growing global concern, yet there has been little research detailing their effects on native ecosystem metabolism and flow hydrodynamics. In this in situ study, we made aquatic eddy covariance oxygen flux and profiling acoustic Doppler velocimetry measurements over high and low density sites within an invasive Gracilaria vermiculophylla (also referred to as Agarophyton vermiculophyllum) macroalgal mat located on a tidal flat along the Virginia (USA) coast. Mean ± SE (n) submerged dark and light oxygen flux over the high density site were -178 ± 9 (144) and 139 ± 26 (104) mmol m-2 d-1, respectively, vs. -71 ± 7 (45) and 32 ± 15 (48) mmol m-2 d-1 over the low density site. The high density site was highly productive, with gross primary production of 169 ± 12 mmol m-2 d-1 (mean ± SE), yet maintained overall metabolic balance, as net ecosystem metabolism was -22 ± 39 mmol m-2 d-1. Close agreement with literature values for air-exposed flux measurements suggests that our results are good first-order approximations of true daily values, demonstrating that this invasion has significantly enhanced tidal flat productivity. Flow and turbulence were significantly attenuated by Gracilaria, resulting in decreased bed shear stress within the algal canopy, likely stabilizing tidal flat sediments and inhibiting mass transport at the sediment-water interface. Turbulence above the canopy was enhanced, but there was no increase in downward momentum transfer. These changes to the hydrodynamic environment decrease the risk of detachment and subsequent advective loss of algae, suggesting a positive feedback that facilitates algal invasion. KEY WORDS: Invasive seaweeds · Macroalgae · Gracilaria vermiculophylla · Oxygen flux · Aquatic eddy covariance · Agarophyton vermiculophyllum · Productivity Full text in pdf format Information about this Feature Article NextCite this article as: Volaric MP, Berg P, Reidenbach MA (2019) An invasive macroalga alters ecosystem metabolism and hydrodynamics on a tidal flat. Mar Ecol Prog Ser 628:1-16. https://doi.org/10.3354/meps13143 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 628. Online publication date: October 10, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Detecting ecological thresholds and tipping points in the natural capital assets of a protected coastal ecosystem

Concern about abrupt and potentially irreversible ecosystem thresholds and tipping points is increasing, as they may have significant implications for natural capital and human wellbeing. Although well established in theory, there are few empirical studies that provide evidence for these phenomena in coastal and estuarine ecosystems, despite their high value for provision of ecosystem services. To determine the likelihood of such events, we tested two statistical methods; sequential T-test analysis (STARS) and generalised additive models (GAMs) in a harbour ecosystem. These methods were applied to time series data spanning up to 25 years coupled with analysis of the relationships between drivers and natural capital asset flows. Results of the STARS analysis identified nonlinear thresholds in three of the natural capital assets of Poole harbour; mudflat area, Manila clam stocks and wader/wildfowl numbers, as well as an increase in several drivers affecting the harbour. The most prominent threshold was recorded in the Manila clam fisheries of the harbour, with stocks in two locations of the harbour declining by 73–78% between 2006 and 2008. We suggest that the historic decline in the Manila clam stocks of the harbour were partly attributable to illegal fishing pressure although other factors such as disease and lease bed holders switching to other species were also likely to have contributed. More recently (2015-onwards) wild clam stocks of the harbour have increased thanks to improved management measures by local authorities. Generalised additive models also identified the contribution of macroalgal mats, sediment shoaling and river flows to historic changes in mudflat area, saltmarsh area and wader/wildfowl numbers. We conclude that information on thresholds and tipping points obtained using these approaches can potentially be of value in a management context, by focusing attention on the interactions and positive feedbacks between drivers that may cause abrupt change in coastal ecosystems.

Environmental factors controlling biomass development of seagrass meadows of Zostera noltei after a drastic decline (Arcachon Bay, France)

The relative impact of light, depth, hydrology, hydrodynamics, sediment and nutrients on growth of the seagrass Zostera noltei was investigated in Arcachon Bay, where the extent of seagrass beds has been decreasing for 20 years. A comprehensive multi-parameter survey of 9 sites was conducted monthly over one year. A three-parameter logistic growth model was fitted to biomass data to unravel the relative contributions of environmental factors on seagrass growth. The present study highlights the fact that, in addition to the dominant positive effect of light on plant growth, the hydrological and hydrodynamic parameters were important drivers for Z. noltei growth at the scale of the whole Bay. At the scale of individual sites, other factors such as macroalgal mats or micro-topography (such as presence of tidal pools) may also influence seagrass growth. The statistical model also suggested that the different factors contribute differently to Z. noltei growth in different sites. Finally, as the model is not able to predict biomass accurately for sites that received very little light, and seagrasses in these sites having higher chlorophyll content and longer shoots, we suspect that adaptation to low light condition was at play. In a global context of seagrass beds decline, our study provides a framework to disentangle the site-specific effects of physical and biological drivers on seagrass seasonal growth.

Do Macroalgal Mats Impact Microphytobenthos on Mudflats? Evidence from a Meta-Analysis, Comparative Survey, and Large-Scale Manipulation

Eutrophication and species introductions have resulted in increased macroalgal biomass in coastal ecosystems around the globe. Macroalgal mats may compete with microphytobenthos (MPB) for light and nutrients and, due to their position in the canopy, have a negative impact on MPB biomass. We tested this effect by conducting a meta-analysis of prior experiments, as well as a comparative survey, and a macroalgal-removal manipulation in the coastal lagoons of the Virginia Coastal Reserve (VCR) on the eastern shore of Virginia (USA). In all cases, MPB biomass was estimated using benthic chlorophyll as a proxy. While prior individual studies documented impacts of macroalgae, when effect sizes were averaged across studies, there was no consistent effect of macroalgal biomass on MPB biomass. In the VCR, a non-native red macroalga, Gracilaria vermiculophylla, dominates intertidal mats and attains high biomasses at some sites. Nevertheless, MPB biomass was unrelated to macroalgal mass based on a survey of mudflats. Further, when macroalgae were experimentally manipulated on a mudflat using a before and after impact design, there was no change in MPB. Based on the meta-analysis, survey, and manipulation we conducted, macroalgal mats do not have a generalizable effect on MPB, interactions seem context-dependent, and in the VCR, the effects on MPB appear neutral. This finding is important given the significance of MPB in supporting food webs and other estuarine ecosystem functions, as well as the increasing frequency and intensity of macroalgal blooms.

Mud-entrained macroalgae utilise porewater and overlying water column nutrients to grow in a eutrophic intertidal estuary

Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters, and accordingly may subsidise growth of benthic macroalgal mats in estuarine ecosystems. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in intertidal mudflat sediments, where access to water column nutrients is tidally constrained. In this study, filamentous Gracilaria chilensis thalli were simultaneously exposed to sediment and overlying water nutrient sources, labelled using 15N tracers (15NH4+ or 15NO3−) during a 5-day experiment. Dissolved inorganic N (DIN) uptake from porewater and overlying water accounted for 33 and 52%, respectively, of the N estimated as necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability (8 h day−1). Of the total N assimilated by the plants, ~ 15% could not be accounted for, supporting the acquisition of other N forms in order to meet demand. We also found that regardless of background NH4+:NO3− ratios (i.e. 1:3 in overlying water and 12:1 in porewater), plants accumulated 15NH4+ significantly more readily than 15NO3−, indicating a preference for NH4+. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive success of entrained macroalgae relative to non-entrained species and historically abundant seagrass beds in these environments. These results underscore the significance of both internal nutrient loading and external inputs as important in sustaining opportunistic macroalgal blooms in shallow estuaries.

Carbon sequestration in macroalgal mats of brackish-water habitats in Indian Sunderbans: Potential as renewable organic resource

Large influx of excess nutrients into sub-tropical brackish-water habitats is expected to radically affect the algal populations in the heavily populated Sunderbans brackish-water ecozone. Twelve selected brackish-water sites in the Indian Sunderbans were surveyed to investigate the growth performance of mat-forming dominant algal/cyanobacterial macrophytes and their potential for carbon (C) sequestration into hydrologic and pedologic pools. The mats were dominated by particular taxa at different seasons related to physico-chemical properties of the wetland habitats. Different environmental variables and biomass productivity parameters were measured on fortnightly basis to assess the carbon cycle related to dominant algal blooms of the study area. The dominating species at the twelve sites included seven genera (Spirogyra, Rhizoclonium, Ulva, Cladophora, Pithophora, Chaetomorpha) belonging to Chlorophyta, three genera (Polysiphonia, Gracilaria, Catenella) belonging to Rhodophyta and Lyngbya majuscula from cyanobacteria. Multivariate statistical methods indicated that nutrient availability, particularly dissolved P concentration and N:P ratio in the water column, along with salinity in the water column mainly affected biomass yield and C sequestration of mat-forming macrophytes and OC input into water column. However, OC contents of underlying muck proved to be very stable, though small influxes of OC occurred at each bloom. High biomass yields (34–3107 g/m2) of the dominant mat components accumulated enormous stocks of OC, very little of which reaches the pedologic pool. This transient biomass might be utilized as dietary supplements or biofuel feedstocks. Availability of important dietary fatty acids in Spirogyra punctulata, Gracilaria sp., Polysiphonia mollis, Rhizoclonium riparium, R. tortuosum, Pithophora oedogonia and Ulva lactuca was considered as suitability of these species as nutraceuticals. Fatty acid compositions of L. majuscula, Catenella repens, R. tortuosum and Cladophora crystallina were estimated to be applicable for producing biodiesel for usage in sub-tropical climates.

Eutrophication decreases salt marsh resilience through proliferation of algal mats

Globally, many estuaries are affected by nutrient loading from human land uses in the surrounding watersheds. One consequence of increased nutrient levels is proliferation of opportunistic macroalgae. We sought to understand spatial and temporal dynamics of ephemeral macroalgal mats and to examine their effects on salt marsh in a eutrophic estuary in central California. A time series analysis spanning 80years revealed that algal wrack has increased exponentially in frequency on the salt marsh, and was highly correlated with nutrient concentrations in the estuary, which have increased along with fertilizer use. Analysis of sediment δ15N showed a dramatic increase in nutrient loads attributable to agricultural fertilizer over the past 50years. We monitored 15 salt marsh plots along the bank edge and detected a negative relationship between algal wrack cover and salt marsh cover, flowering, and canopy height. Moreover, algal wrack led to retreat of vegetation from the bank edge, and increased bank erosion. We also experimentally added algal wrack to salt marsh edge plots. Algal addition decreased salt marsh cover, flowering, and canopy height, and increased retreat rate. By integrating time series analyses, isotope data, algal and marsh monitoring and manipulative experiments, we have identified robust linkages between increased anthropogenic nutrient loading, increased algal wrack cover, reduction in marsh resilience and conversion of marsh habitat to mudflat through bank erosion. Decreasing nutrient inputs to eutrophic estuaries is thus essential for conservation and restoration of salt marshes and enhancing their resilience in the face of sea level rise.

Green tides on inter- and subtidal sandy shores: differential impacts on infauna and flatfish

Beach ecosystems extend from dune to offshore areas along most coasts, and provide essential services that are not provided by any other ecosystem. Indeed, sandy systems contain unique biodiversity and supply nursery and foraging areas for numerous commercially important marine species, such as flatfish. However, these systems are threatened by increasing anthropogenic pressure. Green tides (GT, i.e. accumulations of green opportunistic macroalgae) are a major human-induced threat to marine ecosystems, from inshore to nearshore. This eutrophication process greatly affects both benthic invertebrate communities and flatfish communities, within sheltered and non- or microtidal systems. However, the responses of dynamic open macrotidal sandy systems to eutrophication in the form of macroalgal mats are not yet fully understood. In particular, understanding the effects of GT on two connected biological compartments (infauna and flatfish) within two connected habitats (intertidal and subtidal) is crucial. Here, we set out to assess the influence of several environmental variables, including Ulva biomass, on the variability in infauna and flatfish communities in both the intertidal and the subtidal at four sites impacted or not by GT. In total, 110 biodiversity samples were analysed with classic and novel analytical approaches. Our results demonstrate that the presence of GT specifically impacts intertidal macroinvertebrate communities. However, small effects of GT on subtidal infauna communities, as well as on species-specific flatfish at both intertidal and subtidal, were still detectable. Our findings underline the vulnerability of highly dynamic ecosystems exposed to anthropogenic stress, in particular intertidal sandy shores.

Challenges for the conservation of marine small natural features

Seascapes contain a mosaic of ecosystem types, including ‘small natural features’ that support a diverse fauna and flora, and provide ecosystem services disproportionate to their size. These include tropical coral reefs, seagrass meadows, rhodolith beds, kelp and mangrove forests as well as deep-sea coral reefs, sponge gardens and hydrothermal vents. The contributions of coral reefs, mangrove forests and seagrass meadows are well recognised as important components of tropical marine seascapes. In temperate regions, hotspots of primary productivity such as kelp forests, and vegetation-based ecosystems found at coastal margins play significant roles in nutrient and sediment trapping and coastal protection. Biogenic features in coastal regions can also play important roles, including shellfish reefs, sponge gardens and macroalgal mats that provide physical structure within soft sediment ecosystems. Similar biogenic features are found in the deep sea, which often occur associated with seamounts, canyon features, cold seeps and hydrothermal vents. Many marine small natural features are globally threatened as they are found at the interface with human-influenced landscapes (coastal ecosystems) or are the focus of resource exploitation (e.g., trawling for fish associated with seamounts and canyons). Challenges in management are, however, consistent across depths, and highlight the restricted spatial extent of such features, and the limited information typically available on their distributions, and the ecosystem services they provide. Numerous strategies can be utilized to sustainably manage, protect, create and restore these marine small natural features and their associated biodiversity.

Freshwater population of Ulva flexuosa (Ulvaceae, Chlorophyta) as a food source for great pond snail: Lymnaea stagnalis (Mollusca, Lymnaeidae)

SUMMARYCosmopolitan species of the genus Ulva (Ulvaceae; Chlorophyta) that populate the littoral zone of marine habitats constitute a staple diet for a variety of organisms, particularly snails, shellfish, polychaetes, and birds. Occurrence of Ulva species (e.g., U. flexuosa and U . prolifera) has also been observed in freshwater inland ecosystems that have no contact with saline water. However, the influence of the development of macroalgal mats of Ulva on indigenous organisms in limnic ecosystems has not been established. This study investigates the trophic relationships between Ulva flexuosa and one species of snail from freshwater habitats in central Europe. During the summer, the great pond snail (Lymnaea stagnalis) consumed Ulva as a source of nutrition even when other algae and plants were available. Lymnaea stagnalis consumed an average of 100 mg of Ulva thalli per day. This level of biomass exceeded the consumption of an alternative food source, the shoots of Elodea canadensis. Ulva thalli are more actively consumed by great pond snails than Elodea shoots, and this is expressed in terms of the differences of biomass consumption. It was also observed that the interior of the monostromatic tubular thalli of Ulva flexuosa serves as a protective shelter for juvenile great pond snails.

Artificial soft sediment resuspension and high density opportunistic macroalgal mat fragmentation as method for increasing sediment zoobenthic assemblage diversity in a eutrophic lagoon

Superficial soft sediment resuspension and partial fragmentation of high density opportunistic macroalgal mats were investigated by boat to determine the impact on zoobenthic assemblages in a eutrophic Mediterranean lagoon. Sediment resuspension was used to oxidise superficial organic sediments as a method to counteract the effects of eutrophication. Likewise, artificial decay of macroalgal mat was calculated to reduce a permanent source of sediment organic matter. An area of 9ha was disturbed (zone D) and two other areas of the same size were left undisturbed (zones U). We measured chemical–physical variables, estimated algal biomass and sedimentary organic matter, and conducted qualitative and quantitative determinations of the zoobenthic species detected in sediment and among algal mats. The results showed a constant major reduction in labile organic matter (LOM) and algal biomass in D, whereas values in U remained stable or increased. In the three zones, however, bare patches of lagoon bed increased in size, either by direct effect of the boats in D or by anaerobic decay of the algal mass in U. Zoobenthic assemblages in algal mats reduced the number of species in D, probably due to the sharp reduction in biomass, but remained stable in U, whereas in all three areas abundance increased. Sediment zoobenthic assemblages increased the number of species in D, as expected, due to drastic reduction in LOM, whereas values in U remained stable and again abundance increased in all three zones. In conclusion, we confirmed that reduction of sediment organic load enabled an increase in the number of species, while the algal mats proved to be an important substrate in the lagoon environment for zoobenthic assemblages, especially when mat alternated with bare intermat areas of lagoon bed. Sediment resuspension is confirmed as a management criterion for counteracting the effects of eutrophication and improving the biodiversity of zoobenthic assemblages in eutrophic lagoon environments.

Biomass decay rates and tissue nutrient loss in bloom and non-bloom-forming macroalgal species

Macroalgal blooms occur in shallow, low-wave energy environments and are generally dominated by fast-growing ephemeral macroalgae. When macroalgal mats undergo senescence and decompose they can cause oxygen depletion and release nutrients into the surrounding water. There are relatively few studies that examine macroalgal decomposition rates in areas impacted by macroalgal blooms. Understanding the rate of macroalgal bloom decomposition is essential to understanding the impacts of macroalgal blooms following senescence. Here, we examined the biomass, organic content, nitrogen decay rates and δ15N values for five macroalgal species (the bloom-forming Agardhiella subulata, Gracilaria vermiculophylla, Ulva compressa, and Ulva rigida and the non-bloom-forming Fucus vesiculosus) in Narragansett Bay, Rhode Island, U.S.A. using a litterbag design. Bloom-forming macroalgae had similar biomass decay rates (0.34–0.51 k d−1) and decayed significantly faster than non-bloom-forming macroalgae (0.09 k d−1). Biomass decay rates also varied temporally, with a significant positive correlation between biomass decay rate and water temperature for U. rigida. Tissue organic content decreased over time in all species, although A. subulata and G. vermiculophylla displayed significantly higher rates of organic content decay than U. compressa, U. rigida, and F. vesiculosus. Agardhiella subulata had a significantly higher rate of tissue nitrogen decay (0.35 k d−1) than all other species. By contrast, only the δ15N of F. vesiculosus changed significantly over the decay period. Overall, our results indicate that bloom-forming macroalgal species decay more rapidly than non-bloom-forming species.

The good, the bad and the Ulva: the density dependent role of macroalgal subsidies in influencing diversity and trophic structure of an estuarine community

Worldwide, ecological subsidies enhance ecosystem productivity and therefore trophic support for greater biodiversity of taxa. While studies in terrestrial and aquatic ecosystems demonstrate that the magnitude of subsidies into ecosystems differs widely, the thresholds where subsidies may switch from exerting positive to negative effects are poorly understood. In estuaries, eutrophication promotes drift macroalgae that deposit on the benthos, cover intertidal flats for months and serve as pressed resource subsidies for benthic consumers. We hypothesized there would be a critical threshold of macroalgal biomass where ecosystem‐level effects would turn from positive to negative. We used manipulative field experiments varying macroalgal mat thickness (0.5, 1.5 and 4 cm) over eight weeks and quantified effects on macrofauna on a lagoon mudflat in California. We documented that plots with mat depths of 0.5 and 1.5 cm had higher diversity by supporting both surface feeding and burrowing detritivores. Non‐metric multidimensional scaling showed that the benthic community diverged with mat depth over the course of the experiment. After eight weeks, surface deposit feeders were associated mainly with 0.5 cm macroalgal subsidies, whereas subsurface deposit feeding capitellids were closely linked with 4 cm mats. Depth profiles of pore water sulfide concentration collected from 4 cm mats were 7622 ± 5294 μM, mean ± SE, (mean of means across depth profiles), whereas 0.5 cm treatments resulted in sulfide concentrations that were 0.25% of the 4 cm treatments. This suggests that the mechanism of negative effects for elevated macroalgal subsidies was development of anoxic conditions promoting sulfide accumulation. Thus, our study was the first to find a critical threshold, or ecological tipping point, beyond which the effects of anthropogenically enhanced subsidies to estuarine mudflat communities switched from positive to negative and to describe the mechanism by which elevated subsides altered the abiotic environment and likely reduced ecosystem functioning.

Experimental investigation of the impact of macroalgal mats on the wave and current dynamics

Macroalgal mats of Ulva intestinalis are becoming increasingly common in many coastal and estuarine intertidal habitats, thus it is important to determine whether they increase flow resistance, promote bed stability and therefore reduce the risk of erosion favoring tidal flooding or degradation of coastal lagoons. Venier et al. (2012) [6] studied the impact of macroalgal mats of Ulva intestinalis on flow dynamics and sediment stability for uniform flow. Here we extend their experimental work to the case of vegetation under the combined action of waves and currents. These hydrodynamic conditions are very common in many shallow coastal environments and lagoons. The experimental facility employed in the present study and the series of flow runs are the same as that used by Venier et al. (2012)[6]. However, waves have been superposed to uniform current flowing firstly over a mobile sediment bed covered with U. intestinalis, then over a bare sediment surface. For the depth, wave and current conditions considered in the experiments, the time-averaged vertical profile of horizontal velocity for the case of coexisting waves and current turns out to be very close to that observed for a pure current, both with and without vegetation. However, contrary to what was observed in the case of a unidirectional current, in the presence of waves the time averaged velocity profile is only weakly influenced by the vegetation, whose main effect is to attenuate velocity oscillations induced by waves and to slightly increase the overall bed roughness.