Benthic Microalgae Research Articles

Factors affecting surf zone phytoplankton production in Southeastern North Carolina, USA

The biomass and productivity of primary producers in the surf zone of the ocean beach at Wrightsville Beach, North Carolina, USA, were measured during all seasons, along with environmental parameters and nutrient levels. Variation in biomass (chlorophyll a) was associated with temperature. Primary production (PP), measured by in situ 14-C incubations, was a function of chlorophyll a, tide height at the start of incubations, and rainfall in the preceding 24-hr period. Biomass-normalized production (PB) was also a function of tide height and rainfall in the preceding 24-hr period. We interpreted these results as evidence of surf production 1) as combined contributions of phytoplankton and suspended benthic microalgae, which may confound application of simple P-E models to surf zone production, and 2) being regulated by nutrient source/supply fluctuations independently from other factors. Surf zone biomass and production levels are intermediate between relatively high estuarine values and much lower coastal ocean values. Surf zone production may represent an important trophic connection between these two important ecosystems.

Taiwanese marine microbenthic algal communities remain similar yet chlorophyll a concentrations rise in mesocosms with elevated CO2 and temperature

The effects of increasing CO2 concentrations and temperature on microalgal assemblages were examined in Taiwan using mesocosms that simulate coral reef ecosystem. We assessed changes in abundance and diversity of benthic algae grown at 25°C and 28°C, under ambient (~400μatm) and at high CO2 conditions (800–1000μatm). Total alkalinity, pCO2, and the aragonite saturation state, were all significantly different between control and high CO2 treatments in both temperature treatments. Chl a concentration increased significantly in CO2-treated groups at 25°C, but benthic microalgal abundance was not significantly different. The number of microalgal species and the microalgal community structure did not differ between control and CO2-treated groups at both temperatures. Our results suggest that increasing CO2 may boost benthic microalgal primary productivity if sufficient nutrients are available, although site-specific responses are difficult to predict.

Nutrient and herbivore alterations cause uncoupled changes in producer diversity, biomass and ecosystem function, but not in overall multifunctionality

Altered nutrient cycles and consumer populations are among the top anthropogenic influences on ecosystems. However, studies on the simultaneous impacts of human-driven environmental alterations on ecosystem functions, and the overall change in system multifunctionality are scarce. We used estuarine tidal flats to study the effects of changes in herbivore density and nutrient availability on benthic microalgae (diversity, abundance and biomass) and ecosystem functions (N2-fixation, denitrification, extracellular polymeric substances -EPS- as a proxy for sediment cohesiveness, sediment water content as a proxy of water retention capacity and sediment organic matter). We found consistent strong impacts of modified herbivory and weak effects of increased nutrient availability on the abundance, biomass and diversity of benthic microalgae. However, the effects on specific ecosystem functions were disparate. Some functions were independently affected by nutrient addition (N2-fixation), modified herbivory (sediment organic matter and water content), or their interaction (denitrification), while others were not affected (EPS). Overall system multifunction remained invariant despite changes in specific functions. This study reveals that anthropogenic pressures can induce decoupled effects between community structure and specific ecosystem functions. Our results highlight the need to address several ecosystem functions simultaneously for better ecosystem characterization and management.

Dietary tracers and stomach contents reveal pronounced alimentary flexibility in the freshwater mullet (Myxus capensis, Mugilidae) concomitant with ontogenetic shifts in habitat use and seasonal food availability

We investigated ontogenetic and seasonal variations in the diet of the freshwater mullet (Myxus capensis) across a river–estuary interface using dietary tracer (stable isotopes and fatty acids) and stomach content analyses. Two hypotheses were tested: (A) the freshwater mullet diet shifts as individuals grow and migrate from the estuary to the river, and (B) the dominant food resources utilized by freshwater mullet vary through time, mainly as a function of the seasonal changes in the availability of preferred food items in each habitat. Both hypotheses were supported, as our results indicated broad dietary flexibility by M. capensis, with utilized food items ranging from benthic microalgae to insects depending on habitat and seasonal patterns in availability of resources. Given the unexpected importance of invertebrate-derived prey, including some of terrestrial origin (i.e. aerial or semi-aquatic insects), during the freshwater phase of the M. capensis life cycle, we also emphasize a need for a re-assessment of the trophic designation of this species (previously designated as a strict detritivore).

Uptake of dissolved organic and inorganic nitrogen in microalgae-dominated sediment: comparing dark and light in situ and ex situ additions of 15N

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 571:29-42 (2017) - DOI: https://doi.org/10.3354/meps12127 Uptake of dissolved organic and inorganic nitrogen in microalgae-dominated sediment: comparing dark and light in situ and ex situ additions of 15N Philip M. Riekenberg*, Joanne M. Oakes, Bradley D. Eyre Centre for Coastal Biogeochemistry, Southern Cross University, PO Box 157, Lismore, NSW 2480, Australia *Corresponding author: phrieken@gmail.com ABSTRACT: Microbial communities within bare intertidal sediment have an active role in uptake of inorganic and organic nitrogen as it is transported through estuaries. 15N-labeled dissolved inorganic nitrogen (DIN, NH4+; 250 µmol l-1, 500 µmol l-1) and dissolved organic nitrogen (DON, algal-derived; 125 µmol l-1, 250 µmol l-1) were applied to diatom-dominated sandy intertidal sediment under light and dark conditions to investigate short-term N uptake (24 h). Two experiments compared uptake in intact sediments (in situ) and homogenized slurries (ex situ). In both experiments, N uptake was similar in light and dark conditions, and benthic microalgae (BMA) dominated both biomass and DIN and DON uptake over heterotrophic bacteria. Substantially lower uptake of DON than DIN occurred for both experiments, likely because organic molecules require extracellular processing before uptake by BMA. Compared to intact sediments, sediment slurries had higher N uptake into sediment organic matter (3-36×), lower bacterial biomass (13.6 ± 3.5% versus 41.1 ± 7.6% intact) and low bacterial contribution to 15N uptake (14 ± 0.8% versus 14 ± 3.0%). Differences are likely due to shifts within the microbial community and sediment environment caused by sediment homogenization or incubation effects. Consistently, uptake rates within slurries were greater than within intact sediments, and patterns of significant differences among treatments were different. Slurry incubations are therefore not reliable for quantification or comparison of in situ uptake rates across different N substrates, but biomarkers appear robust between the 2 methods, indicating low bacterial contribution to N uptake in BMA-dominated sediment. KEY WORDS: Benthic microalgae · Bacteria · Microphytobenthos · DON · DIN · Slurry · Stable isotope Full text in pdf format Supplementary material PreviousNextCite this article as: Riekenberg PM, Oakes JM, Eyre BD (2017) Uptake of dissolved organic and inorganic nitrogen in microalgae-dominated sediment: comparing dark and light in situ and ex situ additions of 15N. Mar Ecol Prog Ser 571:29-42. https://doi.org/10.3354/meps12127 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 571. Online publication date: May 17, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Mussel beds are biological power stations on intertidal flats

Intertidal flats are highly productive areas that support large numbers of invertebrates, fish, and birds. Benthic diatoms are essential for the function of tidal flats. They fuel the benthic food web by forming a thin photosynthesizing compartment in the top-layer of the sediment that stretches over the vast sediment flats during low tide. However, the abundance and function of the diatom film is not homogenously distributed. Recently, we have realized the importance of bivalve reefs for structuring intertidal ecosystems; by creating structures on the intertidal flats they provide habitat, reduce hydrodynamic stress and modify the surrounding sediment conditions, which promote the abundance of associated organisms. Accordingly, field studies show that high chlorophyll a concentration in the sediment co-vary with the presence of mussel beds. Here we present conclusive evidence by a manipulative experiment that mussels increase the local biomass of benthic microalgae; and relate this to increasing biomass of microalgae as well as productivity of the biofilm across a nearby mussel bed. Our results show that the ecosystem engineering properties of mussel beds transform them into hot spots for primary production on tidal flats, highlighting the importance of biological control of sedimentary systems.

Primer informe del género <i>Gambierdiscus</i> (Dinophyceae) y otros dinoflagelados bentónicos en el Parque Nacional Isla del Coco, Costa Rica, Pacífico Tropical Oriental

El Pacífico Tropical Oriental se ha indicado como una región de los océanos libre de ciguatera, una intoxicación que causa serios trastornos gastrointestinales, neurológicos y cardiovasculares, en casos extremos, la muerte. Pero con el presente estudio hemos encontrado una incidencia alta de microalgas bénticas involucradas en la producción de ciguatera en el Parque Nacional Isla del Coco (PNIC), Costa Rica. En campañas oceanográficas realizadas entre el 2006 y el 2011 al PNIC se recolectaron 420 muestras de fitoplancton con el interés de encontrar especies de dinoflagelados bentónicos implicados en la producción de ciguatera. Las muestras se tomaron con red de fitoplancton o mediante buceos, entre los 5 y 30 m de profundidad en zonas de arrecife y por extracción directa de macroalgas. Se encontró a los dinoflagelados Gambierdiscus spp., Coolia tropicales, Coolia cf. areolota, Prorocentrum concavum, Prorocentrum compressum, Amphidinium carterae y Ostreopsis siamensis. Se determinó que la cantidad de dinoflagelados por peso de macroalga era elevada, principalmente para el género Gambierdiscus. Los géneros Ostreopsis y Prorocentrum mostraron tener la mayor distribución en los sitios de colecta. Gambierdiscus es un género productor de ciguatera. Se encontraron dos tamaños diferentes de individuos de Gambierdiscus, Comparando los especímenes de Gambierdiscus encontradas con otros estudios, se concluye que son diferentes a las descritas anteriormente, por lo que podría tratarse de especies nuevas, lo que se confirmará con cultivos y análisis moleculares. Al ser la Isla del Coco un sitio oceánico aislado y en estado de conservación, es un sitio ideal para el estudio de la evolución de especies de fitoplancton marino. Además, el monitoreo a largo plazo es muy importante debido a la variedad de dinoflagelados bentónicos potencialmente tóxicos que habitan en este ecosistema marino. Este es el primer estudio donde se da a conocer la incidencia de dinoflagelados bentónicos implicados en ciguatera para el Pacífico Tropical Oriental.

Does reef structure affect oyster food resources? A stable isotope assessment

As ecosystem engineers, oysters create and maintain structured habitat and can influence trophodynamics and benthic-pelagic coupling in the surrounding landscape. The physical reef structure and associated biotic parameters can affect the availability of food resources for oysters. Oysters and potential composite food sources — suspended particulate organic matter (SPOM) and surface sediment organic matter (SSOM) — were assessed using a dual stable isotope (δ13C, δ15N) approach at three reef types (natural, restored, and unconsolidated) seasonally for two years to determine if changes in physical and/or biotic parameters affected the relative availability and/or use of food resources by oysters. SPOM was more depleted in 13C (−24.2 ± 0.6‰, mean ± SD) than SSOM (−21.2 ± 0.8‰). SPOM composition is likely dominated by autochthonous phytoplankton production, while SSOM includes trapped phytoplankton and benthic microalgae. SSOM was used by oysters in increasing proportions relative to SPOM over time at all reef types. This temporal trend is likely due to increased oyster biomass over time, promoting enhanced microphytobenthos growth through feedback effects related to oyster biodeposits. Structural differences between reef types observed in this study had no effect on food resource availability and use by oysters, indicating strong bentho-pelagic coupling likely due to shallow depths as well as strong and consistent winds. This study provides insights for restoration of oyster reefs as it highlights that food resources used by oysters remain similar among reef types despite changes in abiotic and biotic parameters among habitats and over time.

Warming has stronger direct than indirect effects on benthic microalgae in a seaweed system in spring

Using outdoor mesocosms we investigated the relative importance of the direct and indirect (here: altered grazing) effects of seawater warming on benthic microalgae in a Baltic Sea Fucus vesiculosus (Phaeophyceae) system during the spring season. Seawater warming had a positive main effect on microalgal total biomass accrual and growth rate and on total mesograzer abundance and biomass. Moreover, under the existing resource-replete conditions in spring the direct positive effect of warming on microalgae was stronger than its indirect negative effect through enhanced grazing. The outcome of this study contrasts previous observations from the summer and winter season, where indirect effects of warming mediated by altered grazing were identified as an important driver of primary biomass in the Fucus system. In this context, the results from the spring season add mechanistic information to the overall understanding of the seasonal variability of climate change effects. They suggest that the relative importance of the underlying direct and indirect effective pathways of warming and the overall effect on the balance between production and consumption are influenced by the trophic state of the system, which in temperate regions is related to season.

Non-reliance of metazoans on stromatolite-forming microbial mats as a food resource

Grazing and burrowing organisms usually homogenise microalgal mats that form on benthic sediments of many aquatic ecosystems. In the absence of this disruption, microalgal mats can accrete laminated deposits (stromatolites). Stromatolites are rare in modern coastal ecosystems, but persist at locations where metazoans are largely excluded. This study aimed to assess the trophic structure at stromatolite locations where metazoans co-occur, to determine the grazing influence exerted by the metazoans on the stromatolite-forming microalgae (cyanobacteria and diatoms). Stable isotope signatures (δ13C and δ15N) were used as food-web tracers and dietary composition of consumers was calculated using source mixing models. Results clearly demonstrate that the dominant macrofaunal grazers do not utilise stromatolite material as a food resource, but rather subsist on autochthonous macroalgae. For instance, the mean (±SD) dietary composition of two of the most abundant grazers, Melita zeylanica (Amphipoda) and Composetia cf. keiskama (Polychaeta), consisted of 80 ± 11% and 91 ± 7% macroalgae, respectively. This suggests that the stromatolite-forming benthic microalgae are not disrupted significantly by grazing pressures, allowing for the layered mineralisation process to perpetuate. Additionally, grazers likely have a restrictive influence on pool macroalgae, maintaining the competitive balance between micro- and macroalgal groups.

First record of two potentially toxic dinoflagellates in tide pools along the Sardinian coast

Nowadays harmful algal blooms (HABs) represent a serious problem for the conservation of the biodiversity in the Mediterranean Sea. Nevertheless the knowledge on the presence of potentially toxic benthic microalgae in particular habitats, such as tide pools, is still scarce. In order to detect HAB-producing benthic microalgae in tide pools of the rocky intertidal zone, a pilot study was conducted in Tavolara Punta Coda Cavallo Marine Protected Area (MPA) during the late spring of 2016. Three different pools were sampled in two study sites (six pools were sampled in total) and the cell density of toxic species was estimated in each. In all the collected samples, the two potentially toxic dinoflagellates, Prorocentrum lima (Ehrenberg) F. Stein and Coolia monotis Meunier, were recorded and significant differences in their density were observed, in relation to both sites and pools.

Changes in Sediment Characteristics upon Oyster Reef Restoration, NE Florida, USA

As ecosystem engineers, oysters alter a host of physical and chemical characteristics of their environment. Oyster reef construction (or restoration) can therefore be expected to have wide ranging effects, from increased refuge for organisms, to lower turbidity. We measured changes in sediment characteristics and benthic nutrient fluxes near a constructed intertidal oyster reef in the Guana Tolomato Matanzas National Estuarine Research Reserve in NE Florida, USA. Within one year of reef construction, particle size distribution shifted towards finer sediment, and organic matter content increased to 7.3% ± 2.1 (standard error) compared to 2.9% ± 0.8 (standard error) at control sites, where oysters were absent. After three years, up to 15 cm of this fine, organic-rich sediment had accreted immediately shoreward of the reef. In benthic flux experiments performed under dark conditions, reef sediments released 167 μM/m2/hr NH4+ compared to –4 μM/m2/hr from control site sediments. Under light conditions both reef and control fluxes were insignificant. This was presumably due to uptake by benthic microalgae; chlorophyll a concentrations were 3 times higher in reef sediments compared to controls. Deposition of organic rich sediment near oyster reefs might render important ecosystem services from carbon burial and perhaps increased denitrification. However, burial of live oysters with excessive sediment accretion might threaten reef success in some cases. Our results highlight the complexity of physical-biological interactions and the need for complete assessment of ecosystem services in order to optimize design and thoroughly evaluate restoration outcomes.

Regulation in the Metabolism and Community Structure of a Tropical Salt Flat after Rainfall

ABSTRACT Bento, L.; Masuda, L.S.M.; Peixoto, R.B., and Enrich-Prast, A., 2017. Regulation in the metabolism and community structure of a tropical salt flat after rainfall. Tropical salt flats typically lack a water column for most of the year, which means that rainfall is probably one of the major factors that regulate benthic microalgae and metabolism in areas subjected to periodic drought. Therefore, the goal of this study was to evaluate the effects of rainfall on the ecological function and community structure of a tropical mangrove salt flat area. This study showed that the highest primary production and respiration fluxes were recorded on the last day of sampling when it rained (−7.6 and 4.7 mmol C-CO2 m−2 h−1, respectively). Net primary production increased significantly compared with the dry period that preceded the rain event. The results also suggested that community structure was regulated by rainfall. After the rain event, abundance increased by one order of magnitude, but the diversity and ev...

Quantitative contribution of primary food sources for a mangrove food web in Setiu lagoon from East coast of Peninsular Malaysia, stable isotopic ([formula omitted] and [formula omitted]) approach

Applying stable isotopes to explore insight into the food web structure in marine ecology have been increasing recently. Tropical mangrove forest, one of the vital marine habitats, has been received intensive attentions in the field. However, there is still controversy about the important roles of primary carbon sources provide to consumers in the ecosystem. In this study, stable isotopic (13C and 15N) ratios were used to determine food web and to estimate quantitative contributions of primary carbon sources for invertebrates and four fish species in the tropical mangrove from Setiu lagoon, Malaysia using the Bayesian mixing model (SIAR). The primary nutrient sources showed δ13C values ranging from −18.7 to −31.1‰ and δ15N from 2.5 to 3.8‰. The wide range of the isotopic composition of consumers (−26.1 to −18.4‰ and 4.3 to 10.2‰ for δ13C and δ15N, respectively) indicated that a variety of carbon sources provided to the intertidal mangrove food webs. Mixing model outputs indicated that mangrove-derived carbon was important food sources for gastropod, Littoraria carinifera, Neritina violacea, N. coromandeliana, N. cornucopia, Cerithdea quadrata, and sesarmid crab, while the benthic microalgae (BMA) mainly fueled to Cerithidea djadjariensis, Cerithium coralium, fiddler crab and the crenate crab Thalamita crenata. Two Scylla species relied largely on BMA and SOMs. Bivalve species mainly assimilated phytoplankton-derived carbon, however, the difference of stable carbon isotope ratios among these bivalve species might relate to feeding selectivity or a small portion of BMA contributed to their diets. The model also resulted that BMA and seston sources might be important to the studied fish species rather than mangrove-derived carbon sources. However, further studies are needed to elucidate insight into the fish food web, and to determine whether there is ecological connectivity between habitats inside and outside of the lagoon.

Nitrogen uptake and coupled nitrification–denitrification in riverine sediments with benthic microalgae and rooted macrophytes

Riverine ecosystems receive nitrogen loads from point and diffuse sources that are transferred downstream. Such loads may undergo poorly explored retention and dissipation processes, varying along gradients of nitrogen availability due to different interactions among primary producers and microbial communities. We measured carbon uptake and nitrogen fluxes in microcosms containing riverine sediments with benthic algae and submerged macrophytes (Vallisneria spiralis L.). Coupled nitrification–denitrification rates were determined via 15N–NH4 + injection in the pore water and quantification of the produced 29N2 and 30N2. Two sites with different N–NO3 − concentration and sediment organic content were investigated. We hypothesized that: (1) N–NO3 − availability promotes water column N uptake and attenuates primary producers-bacteria competition; (2) coupled nitrification–denitrification is stimulated by radial oxygen loss from roots; (3) macrophyte meadows favour both temporary nitrogen retention and permanent removal. Sediments with V. spiralis were mostly inorganic C and N sinks and always displayed higher coupled nitrification–denitrification rates compared to sediments with microphytobenthos. Highest rates, up to 100 µmol N m−2 h−1, were measured at the more eutrophic site and in the light. This is likely due to a shift from root to leaf uptake at the N–NO3 − rich site, attenuating plant-bacteria competition, and to increased radial oxygen loss in organic-rich sediments, stimulating nitrification. High rates of N uptake and loss in lotic sediments represent natural buffers preventing N transport downstream and stress the need to preserve aquatic vegetation and its ecosystem services.

Peritidal stromatolites at the convergence of groundwater seepage and marine incursion: Patterns of salinity, temperature and nutrient variability

Living peritidal stromatolites forming at the interface of coastal groundwater seepage and regular marine input are known from only a few locations globally, including South Africa, Western Australia and Northern Ireland. In contrast to modern stromatolites from exclusively fresh or marine waters, which persist due to high calcium carbonate saturation states or hypersaline and erosive conditions (which exclude organisms that might disrupt or out-compete the stromatolite-forming benthic microalgae), the factors supporting stromatolite formation at peritidal locations have not been well-documented. Therefore, the aim of this study was to investigate the fine-scale physico-chemical parameters in terms of pool temperature, salinity and nutrient dynamics at three representative sites along the coastline near Port Elizabeth, South Africa. These parameters were assessed with reference to potential physical, meteorological and ocean drivers using a linear or linear mixed-effects modelling approach. Results demonstrate that nutrient inputs into the pools supporting the majority of stromatolite accretion (barrage pools) are driven by groundwater seepage site-specific properties related to anthropogenic occupation (dissolved inorganic nitrogen; DIN) as well as marine water incursion (dissolved inorganic phosphorus; DIP). Pool temperature is a function of seasonal ambient variability while salinity reflects regular state shifts from fresh to marine conditions, which are related to tidal amplitude and swell height. The regular marine incursions likely promote benthic primary biomass in the phosphorus-limited stromatolite pools, as well as preclude organisms which might otherwise outcompete or disrupt the stromatolite microalgae due to intolerances to extreme (~1.5 to ≥30) salinity variability.

Trophic ecology of Atlantic seabob shrimp Xiphopenaeus kroyeri: Intertidal benthic microalgae support the subtidal food web off Suriname

A combination of stomach content analyses and dual stable isotope analyses was used to reveal the trophic ecology of Atlantic seabob shrimp Xiphopenaeus kroyeri off the coast of Suriname. This coastal penaeid shrimp species has a rather omnivorous diet, feeding opportunistically on both animal prey and primary food sources. The species is a predator of hyperbenthic crustaceans, including copepods, amphipods and the luciferid shrimp Lucifer faxoni, which are mainly preyed upon during daytime, when these prey typically reside near the seabed. Benthic microalgae (BM) from intertidal mudflats and offshore sedimentary organic matter (SOM) were important primary food sources. Due to their depleted 13C values, coastal sedimentary and suspended organic matter, and carbon from riverine and mangrove-derived detritus were not incorporated by X. kroyeri. An ontogenetic diet shift was observed from postlavae to juveniles and adults. Adult X. kroyeri were located higher in the food chain, mainly preying on larger benthic organisms. Intertidal BM were an important food source for all life stages of X. kroyeri, contributing up to 64% to the overall diet based on a Bayesian mixing model. Because X. kroyeri is the main epibenthic organism found at high densities in nearshore waters up to 30 m depth, the species plays a crucial role in transferring energy from low trophic level prey and primary food sources up to higher levels in the food chain. Our results indicate that primary production on intertidal mudflats, through BM, forms an important energy source for the subtidal turbid-water food web in muddy tropical coasts. Conservation of intertidal areas and their associated mangrove systems will therefore likely benefit coastal shrimp production and fisheries in tropical ecosystems.

Neutral processes and species sorting in benthic microalgal community assembly: effects of tidal resuspension

Benthic microalgae (BMA) provide vital food resources for heterotrophs and stabilize sediments with their extracellular secretions. A central goal in ecology is to understand how processes such as species interactions and dispersal, contribute to observed patterns of species abundance and distribution. Our objectives were to assess the effects of sediment resuspension on microalgal community structure. We tested whether taxa-abundance distributions could be predicted using neutral community models (NCMs) and also specific hypotheses about passive migration: (i) As migration decreases in sediment patches, BMA α-diversity will decrease, and (ii) As migration decreases, BMA community dissimilarity (β-diversity) will increase. Co-occurrence indices (checkerboard score and variance ratio) were also computed to test for deterministic factors, such as competition and niche differentiation, in shaping communities. Two intertidal sites (mudflat and sand bar) differing in resuspension regime were sampled throughout the tidal cycle. Fluorometry and denaturing gradient gel electrophoresis were utilized to investigate diatom community structure. Observed taxa-abundances fit those predicted from NCMs reasonably well (R2 of 0.68-0.93), although comparisons of observed local communities to artificial randomly assembled communities rejected the null hypothesis that diatom communities were assembled solely by stochastic processes. No co-occurrence tests indicated a significant role for competitive exclusion or niche partitioning in microalgal community assembly. In general, predictions about relationships between migration and species diversity were supported for local community dynamics. BMA at low tide (lowest migration) exhibited reduced α-diversity as compared to periods of immersion at both mudflat and sand bar sites. β-diversity was higher during low tide emersion on the mudflat, but did not differ temporally at the sand bar site. In between-site metacommunity comparisons, low- and high-resuspension sites exhibited distinct community compositions while the low-energy mudflats contained higher microalgal biomass and greater α-diversity. To our knowledge this is the first study to test the relevance of neutral processes in structuring marine microalgal communities. Our results demonstrate a prominent role for stochastic factors in structuring local BMA community assembly, although unidentified nonrandom processes also appear to play some role. High passive migration, in particular, appears to help maintain species diversity and structure communities in both sand and muddy habitats.

Inhibitory effect of polyunsaturated aldehydes (PUAs) on the growth of the toxic benthic dinoflagellate Ostreopsis cf. ovata

Diatoms have been shown to produce and release a wide range of secondary metabolites that mediate interactions between individuals of different species. Among these compounds, different types of fatty acid derived long-chained polyunsaturated aldehydes (PUAs) have been related to multiple functions such as intra- or interspecific signals and adverse effect on the reproduction of marine organisms. Several studies have reported changes on growth, cell membrane permeability, flow cytometric properties and cell morphology in phytoplankton organisms exposed to PUAs, but little information is available on the effect of these compounds on benthic microalgae. Ostreopsis cf. ovata is a toxic benthic dinoflagellate which causes massive blooms along the Mediterranean coasts typically during the late summer period. In this study the effects of three toxic PUAs known to be produced by several algae (2E,4E-decadienal, 2E,4E-octadienal and 2E,4E-heptadienal) on the growth, cytological features and cell morphology of O. cf. ovata were investigated. Our results show a clear decrease of O. cf. ovata growth with longer-chain molecules than with shorter-chain ones, confirmed also by EC50 values calculated at 48h for 2E,4E-decadienal and 2E,4E-octadienal (6.6±1.5, 17.9±2.6μmolL−1 respectively) and at 72h for 2E,4E-heptadienal (18.4±0.7μmolL−1). Moreover, morphological analysis highlighted up to 79% of abnormal forms of O. cf. ovata at the highest concentrations of 2E,4E-decadienal tested (9, 18 and 36μmolL−1), a gradual DNA degradation and an increase of lipid droplets with all tested PUAs. Further studies are needed to better clarify the interactions between diatoms and O. cf. ovata, especially on bloom-forming dynamics.

Phytoplankton community dynamics within peritidal pools associated with living stromatolites at the freshwater–marine interface

Recently-discovered peritidal stromatolite ecosystems in South Africa form at the interface of freshwater seeps and the ocean intertidal zone, sharing several similarities with both tidal pool and estuarine ecosystems. While the overall ecology of tidal rock pools has been well studied, the dynamics of the phytoplankton assemblage have been comparatively neglected. In addition, there are no studies to date which describe the dynamics of phytoplankton within a habitat associated with stromatolites. The aim of this study was to investigate the coarse-scale phytoplankton community composition of a series of peritidal pools associated with living stromatolites, using a spectral fluorescence analysis tool, in relation to source-specific drivers related to both freshwater and marine forces. Three sites were sampled monthly from January to December 2014. Physico-chemical, biotic and meteorological parameters were recorded to assess some of the factors which might influence the phytoplankton size-fractionation and community composition using a generalised linear modelling approach. Results indicate that fresh or marine pool state, temporal differences associated with season, macronutrients (N and P), and benthic microalgal biomass are important drivers of the phytoplankton assemblages. Specifically, a transition from fresh to marine pool conditions resulted in an increased abundance of smaller phytoplankton size fractions and a shift from Chlorophyta and Cyanophyta to Bacillariophyta and Cryptophyta. Overall, the community was dominated by Chlorophyta and Bacillariophyta. There was consistency between the drivers and composition of the phytoplankton community compared to those from the few other comparable published studies. Furthermore, this study demonstrates a system which is dominated by benthic rather than pelagic microalgae in terms of biomass, thereby supporting the persistence of actively accreting stromatolites.