Abundance Of Microphytobenthos Research Articles

Carbon Dioxide Fluxes from Peri-urban Tidal Flats in South China

Coastal ecosystems are recognized as important carbon reservoirs. However, assessments of coastal carbon dioxide (CO2) fluxes often neglect unvegetated tidal flats, particularly in peri-urban areas. This study investigated the gross primary production (GPP), net primary production (NPP), and total respiration (TR) of three tidal flats in Hong Kong, one of the largest cities in South China, during both summer and winter seasons to understand their CO2 fluxes. Results showed that GPP of three tidal flats was significantly higher in winter than in summer. However, no significant seasonal variations in TR were observed among the tidal flats. Structural equation modelling was used to examine the drivers of CO2 fluxes in the three tidal flats. The model showed that temperature and microphytobenthos abundance were identified as positive drivers of GPP, while sediment mud content had a positive effect on TR. The estimated daily NPP of these tidal flats ranged from -0.853 to 0.112 g C m−2 d−1, which is lower than the mean value reported for global vegetated coastal wetlands. Despite some seasonal and spatial variations, those peri-urban tidal flats may be considered as weak CO2 sources rather than CO2 sinks.

The Microphytobenthos are Abundant and Mediate Key Carbon Fluxes in Tropical Mangroves

AbstractThe microphytobenthos (MPB) are known to play important ecological roles in estuarine ecosystems. While tidal flat MPB are widely studied, mangrove MPB are often overlooked due to the common belief that the light-limited environment under the mangrove canopy is not conducive to MPB growth. We assessed the biomass and community characteristics of the MPB in two contrasting mangrove forests (silty substrate and brackish influence versus sandy substrate and oceanic influence), their seasonal photosynthetic performance, as well as their role in mediating porewater carbon flux. Biomass measured as chlorophyll a concentration indicated that MPB abundance was equal, if not higher, in mangroves than on tidal flats. While mangrove sediments fostered an equally rich assemblage of MPB (estimated 23 to 45 genera) as the adjacent tidal flat, the MPB genus composition was different across all habitats, sites, and seasons, indicating the strong environmental influence on MPB assemblages. This study identified porewater salinity, temperature, and pH, as well as irradiance level and surface soil temperature to be accountable for 23.5% of this variation. MPB in all sampling locations were dominated by pennate diatoms, while cyanobacteria were common in the estuarine (as opposed to oceanic) site. Compared to tidal flat MPB, mangrove MPB exhibited photosynthetic performance characteristic of lower light acclimation. A mesocosm experiment using enriched stable isotopes of carbon and nitrogen indicated that a tight coupling exists between mangrove-derived carbon and MPB. Mangrove MPB are abundant and different from tidal flat MPB, and are functional photosynthetic entities with a regulatory role in nutrient cycling in mangrove ecosystems.

Effect of tidal resuspension with oyster biodeposits on nutrient and oxygen dynamics

To test the effect of biodeposit resuspension on nutrient and oxygen dynamics, we performed a 30 d experiment in three 1000 l shear turbulence resuspension mesocosm (STURM) tanks (R) and three 1000 l non-resuspension (NR) tanks. All tanks contained defaunated muddy sediment and brackish estuarine water, received daily additions of oyster biodeposits, and had similar water column root mean square turbulent velocities (~1 cm s-1), energy dissipation rates (~0.08 cm2 s-3), and tidal cycles (4 h mixing-on; 2 h mixing-off). While bottom shear stress was low in NR tanks, high instantaneous bottom shear produced sediment and biodeposit resuspension in R tanks during the mixing-on cycles. Resuspension and biodeposit addition resulted in complex nutrient and oxygen dynamics in the water column as well as altered seabed fluxes. Modeled biodeposit diagenesis demonstrated that added resuspended biodeposit nitrogen in R tanks was nitrified, resulting in high water column nitrate and nitrite concentrations, as well as increasing water column dissolved oxygen demand. Water column dissolved oxygen concentrations were 2.3 times lower in the R tanks than in the NR tanks, whereas deposited organic matter from biodeposits resulted in sediment dissolved oxygen uptake 3 times higher in NR versus R tanks. Sediment dissolved inorganic nitrogen uptake in NR tanks and efflux in R tanks, respectively, were mediated by microphytobenthos abundance and biodeposit deposition. Seabed and water column biogeochemical processes, as mediated by biodeposit resuspension, controlled the nutrient and oxygen balances. Biodeposit resuspension is important when evaluating oysters as support for eutrophication control.

Spatial and temporal variability of microphytobenthos in a marine hypersaline lake (Crimea): Are there some general patterns?

The microphytobenthos is one of the key elements in the marine hypersaline water bodies. To find the general regularities of microphytobenthos distribution patterns in marine hypersaline lakes and lagoons, the accumulation of data on various cases is needed. Studies on species composition and abundance of microphytobenthos were conducted in a Crimean closed hypersaline lagoon at three stations from April to September 2018. Floating mats of the filamentous green algae Cladophora and the benthic community were investigated. Salinity varied from 23 to 170 g L−1. In total, 25 microalgal species were recorded in the mats and on the bottom, including Bacillariophyta – 19 species, Haptophyta – 2 species, Miozoa – 2 species, and Cyanobacteria – 2 species. 17 Bacillariophyta species were recorded in the mats, and 5 species at the bottom. In the mats, Bacillariophyta species contributed from 50 to 100% of the total species richness. The total microalgal abundance in the mats varied from 2.5 × 106 cells m−2 to 3.8 × 108 cells m−2 (on Cladophora filament area). In 43% of all bottom samples, there was no microphytobenthos at all or there were only single cells, due to hypoxic or even anoxic conditions under the floating mats. The average microalgal abundance in the soft-bottom community was very low, on average 4.1 × 108 cells m−2 of the bottom area. Data on fluctuations of the total microalgae abundance in mats showed that temporal variability is higher than spatial. The factor of chance is important in the determination of microphytobenthos structure variability.

Microphytobenthos in the Hypersaline Water Bodies, the Case of Bay Sivash (Crimea): Is Salinity the Main Determinant of Species Composition?

In hypersaline water bodies, the microphytobenthos plays a very important ecosystem role and demonstrates variability along with a salinity change. Due to anthropogenic activity, the sharp salinity increase in Bay Sivash occurred after 2014. To assess the changes in the microalgae community during the bay ecosystem transformation, the study was conducted four times in 2018 and 2019. At every sampling period, the samples were taken in a salinity gradient (from 7 to 10 sites). A total of 40 species of microalgae were identified during all research, including Cyanobacteria (Cyanophyceae, 2 species), Ochrophyta (Bacillariophyceae, 35 species), Haptophyta (Prymnesiophyceae, 2 species), and Miozoa (Dinophyceae, 1 species). According to the calculated similarity indices of Jaccard and Czekanowski–Sørensen–Dice, the species composition significantly differed during sampling periods. A total of 15 species were recorded at salinities of 80–90 psu, and 10 species at higher salinities, which contribute 64% of all species found in this study. The microalgae abundance was two times more in the floating green algae mat than on the bottom. There was no significant correlation between the number of species and salinity in all sampling periods. In November 2018, a significant positive correlation between the number of species in the sample and total suspended solids (TSS) and dissolved organic matter (DOM) was revealed. A significant correlation between the cell length in different species and salinity and DOM concentration was noted. Before the onset of the salinity increase, 61 species of microalgae were found in Eastern Sivash, of which only 12 have now been recorded, 31% of the currently found species. The characteristics of the total microphytobenthos abundance also significantly changed during all studies. Many characteristics have changed in the bay: the concentration of total suspended matter and dissolved organic matter, the temperature regime, composition of zoobenthos and plankton, and oxygen concentration. Due to this, it is unlikely that only the salinity increase caused the microphytobenthos changes in the lagoon.

Influence of microphytobenthos on the sedimentary organic matter composition in two contrasting estuarine microhabitats.

The influence of microphytobenthic community on the composition of sedimentary organic matter was evaluated in two Bahía Blanca estuary microhabitats contrasting in vegetation and hydrodynamic conditions; namely, S1 located in a Sarcocornia perennis saltmarsh with macroscopic microbial mats and S2 in a mudflat without macroscopic microbial mats or vegetation. Moisture, organic matter, chlorophyll a, phaeopigments, carbohydrates (CH), proteins (PRT), and abundance and structure of microphytobenthic community were evaluated in surface sediments. Higher moisture was observed at S2 and was related to the proximity of this site to the subtidal zone and the effect of the environmental variables temperature and rain. No significant differences were found in organic matter content between sites; however, at S1, a higher concentration was registered during winter and early spring associated to the period of higher microphytobenthic biomass. Chlorophyll a and phaeopigments were higher at S1, attributed to the higher microphytobenthos abundance at this site. Differences in microphytobenthos were observed not only in quantity but also in community structure since at S1 filamentous cyanobacteria dominated the community, whereas at S2, higher abundance of centric diatoms and the absence of cyanobacteria were observed during most of the study. S1 showed higher concentration of proteins and carbohydrates which could be attributed to the higher production of fresh organic matter by microphytobenthos. The total protein and carbohydrate concentrations allowed us to classify both microhabitats into the meso-oligotrophic category, contrasting with the classification made by other authors using water column proxies.

Microbenthic Net Metabolism Along Intertidal Gradients (Cadiz Bay, SW Spain): Spatio-Temporal Patterns and Environmental Factors

The changes in the annual biomass distribution of microphytobenthos (MPB) have been addressed by different studies, although this information is scarce in southern European coastal systems when compared to Northern European estuaries. Moreover, there is little information regarding annual changes in the primary production of MPB related with those in the annual biomass. For this reason, we followed the net production in light, dark respiration rates (determined by oxygen microelectrodes) and MPB abundance (estimated by sediment chlorophyll a) along an intertidal gradient of the inner Cadiz Bay during an annual cycle. Sediment cores were collected seasonally along two transects (5 sampling stations per transect) with contrasting sediment granulometry: muddy (Puerto Real, PR) and muddy-sandy (San Fernando, SF). Our results show that MPB biomass and net production increased seawards reaching maxima in winter. The seasonal maximum of daily solar irradiance and high temperature reached in summer in southern Spain likely inhibits MPB production. Spatial patterns of MPB biomass and net production depend on season and environmental heterogeneity. Environmental variables, analyzed by principal component analyses, were used to explain the variability of MPB metabolism by multiple regression. The principal component axes explained up to 60 % of the microphytobenthic net production and 41 % of the respiration rates in the PR transect while they only explained about 25 % of the same rates in SF. The variability in the environmental variables contributing to the observed patterns implies that no single set of variables can be used to predict net production patterns in the entire system. The different patterns of net production observed between transects highlight the importance of considering the spatial heterogeneity of our system to estimate the contribution of MPB to the inner Cadiz Bay productivity. This production is of major importance since our data showed that microphytobenthic production can contribute up to 49 % of the total benthic production of intertidal sediments of the inner Cadiz Bay.

Correction to: Spatial Variation of Macroinfaunal Communities Associated with Zostera marina Beds Across Three Biogeographic Regions in Atlantic Canada

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.

The State of Benthic Diatom Communities in Listvennichnyi Bay of Lake Baikal (Russia)

The coastal algocenosis of the Listvennichnyi Bay of Baikal since 2011 has been undergoing changes because of excess anthropogenic level. We studied species composition and abundance of the bay littoral diatom community in the spring and summer of 2016-2017 along a section located at a depth of 2-8 m. In the studied period, Fragilaria capucina (Kutz.) J.B. Petersen dominated the studied area. The abundant development of this taxon (yielding than 50% of the total number of microalgae) and associated species of diatoms in the spring decreased the number of microphytobenthos to extremely high, previously unrecorded values. Inhibition of the development of littoral endemic species was noted; however, the endemic Hannaea baicalensis Genkal, Popovskaya et Kulikovskiy dominated at depth of 2 m with numbers comparable with F. capucina. It happened firstly at the beginning of ecological degradation of the coastal zone in 2011, when the development of this and other Baikal endemics fell to solitary cells. The subdominant complex of benthic diatoms included frequently occurring, mainly mesosaprobic species. Changes in the dominant complex of diatom communities together with the high abundance of microphytobenthos indicate a steady trend of increasing water trophic level and the further development of eutrophication in the Listvennichnyi Bay.

Benthic microalgae community response to flooding in a tropical salt flat.

This research evaluated the effect of flooding on the microphytobenthos community structure in a microbial mat from a tropical salt flat. Field samples were collected during four consecutive days: on the first three days the salt flat was dry, on the fourth day it was flooded by rain. In order to evaluate the community maintained in flood conditions, samples from this area were collected and kept in the laboratory for 10 days with sea water. The results of total abundance of microphytobenthos varied from 4.2 × 108 to 2.9 × 109 organisms L-1, total density increased one order of magnitude under the effect of water for both situations of precipitation in the salt flat and in experimental conditions, an increase due to the high abundance of Microcoleus spp. Shannon index (H') was higher during the desiccation period. Our data suggest that changes in the abundance of organisms were due to the effect of water. The dominance of the most abundant taxa remained the same under conditions of desiccation and influence of water, and there is probably a consortium of microorganisms in the microbial mat that helps to maintain these dominances.

Predicting distribution of microphytobenthos abundance on a reef platform by combining in situ underwater spectrometry and pigment analysis

An approach is presented to predict microphytobenthos (MPB) abundance at Heron Reef, Australia using a regression model between chlorophyll-a (chl-a) concentration and field measured spectral reflectance integrated to the blue spectral band of the Worldview-2 (WV-2) sensor. In situ underwater spectral reflectance was measured from benthic sediments and co-located sediment cores were collected to estimate their chl-a concentration. Chl-a features were visually identified in spectral reflectance curves by derivative analysis. Pigment composition analysis of benthic sediments enabled the presence of a relatively homogeneous and living MPB community to be inferred. Field spectral reflectance and chl-a concentration showed a negative correlation in blue and red wavelengths. Regression models between field spectral reflectance resampled to the WV-2 sensor’s bandwidths and chl-a concentration showed the highest coefficient of determination in the blue spectral band of the sensor. This ‘blue band’ model can be further applied to individual pixels of WV-2 images to produce spatially explicit and continuous maps of chl-a concentration, and hence MPB abundance, in Heron Reef.

Seasonal variation in physical and biological factors that influence sediment porosity on a temperate mudflat: Willapa Bay, Washington, USA

A field study of a temperate mudflat in Willapa Bay, Washington was conducted to determine whether seasonal variation in physical forcings (e.g., temperature, rainfall, insolation) coupled with variation in key biological properties (e.g., seagrass and microphytobenthos abundance) would result in measurable changes in seabed porosity. On two mudflats and in a nearby secondary channel, replicate samples for surficial grain size, chlorophyll a content, and seagrass (Zostera japonica) shoot density were obtained at roughly 7–wk intervals for one year starting in November 2009. In addition, high-resolution profiles of porosity in the upper ∼9cm of the seabed were measured using an in situ resistivity profiler. Ancillary meteorological and river discharge data were obtained from nearby stations operated by Washington State University and the U. S. Geological Survey, respectively. Despite significant seasonal variation in seagrass abundance (0 to >3000 shoots m−2), a less dramatic, but still significant variation in chlorophyll a content, as well as seasonal changes in physical forcings, the near-surface porosity of the flats was a normally consolidated bed that remained steady throughout the year. In contrast, porosity of the channel often departed from a normally consolidated profile, with evidence for both erosion and deposition of several centimeters of sediment. These departures did not display a clear seasonal pattern (i.e., deposition in winter, erosion in summer), however, suggesting that the provision and removal of sediment to/from the channels was governed by processes (e.g., channel slumps, rainfall events) occurring on shorter time scales or that there was significant, unresolved small-scale variability.

Coupling between microphytobenthic biomass and fiddler crab feeding

Among the organic matter ingested by fiddler crabs, microphytobenthos is of fundamental importance because it is their main N source. Microphytobenthos abundance generally develop semilunar changes as the dynamics of tidal exposures and day-night cycle are not held constant across days, modifying the balance between growth and mortality. In this study we explored the coupling between temporal dynamics in microphytobenthos abundance and crab feeding activity. We measured the Chlorophyll a content in the 2mm surficial sediment surrounding the burrows and the crab feeding activity over two semilunar cycles. Chlorophyll a and crab feeding activity showed biweekly cyclic dynamics. Crabs did not concentrate feeding activity around days with maximum abundance of microhytobenthos. This phase difference between both dynamics could be the result of the crab feeding impact, but a crab experimental exclusion showed that the temporal dynamics of Chlorophyll a content stayed unchanged when feeding activity was removed. Comparisons between fed and unfed sediment suggest that the feeding efficiency changes with tidal dynamic. Crabs achieved more than 50% of Chlorophyll a extraction during days of highest feeding activity, and less than 30% during days of low feeding activity or low microhytobenthos abundance. Furthermore, comparisons of fed sediment between consecutive days indicated that Chlorophyll a was completely replenished during days with high flooding tides, but partially replenished during days near neap tides. Environmental conditions affecting feeding efficiency may select crabs to concentrate feeding activity before days with the highest microhytobenthos abundance. The low feeding impact on microphytobenthos dynamics suggests that fiddler crabs would not control microhytobenthos abundance and thus unable to absorb the increasing eutrophication of studied estuarine areas.

Effects of macroalgal blooms on carbon and nitrogen biogeochemical cycling in photoautotrophic sediments: An experimental mesocosm

The effects of floating macroalgae (250 g DW m(-2)) on photoautotrophic microbenthos were studied in a flow-through mesocosm with a parallel mesocosm without macroalgae serving as Control. Vertical microprofiles of O(2) at the sediment-water interface showed a immediate and complete suppression of photosynthetic activity of microphytobenthos (MPB) under the macroalgal canopy, resulting in a shift of benthic metabolism from autotrophic to heterotrophic. MPB abundance and chlorophyll a content decreased and a change from a diatom-dominated to cyanobacteria-dominated community was observed. Inorganic nitrogen nutrients' concentrations increased in the porewater as a result of the inhibition of MPB nutrient demand, leading to an increase in net ammonification and anaerobic NO(x)(-) consumption rates. No organic matter transfer from macroalgae to the sediment was detected, resulting in a net consumption of the carbon and nitrogen stored in the sediment. In consequence, sediment was progressively impoverished in nitrogen, reducing sediment nutrient regeneration.

Artificial Reefs Concentrate Nutrients and Alter Benthic Community Structure in an Oligotrophic, Subtropical Estuary

The construction of artificial reefs in the oligotrophic seagrass meadows of central Florida Bay attracted large aggregations of fish and invertebrates, and assays of nutrient availability indicated increases in availability of nutrients to sediment microalgae, periphyton, and seagrasses around reefs. An average of 37.8 large (> 10 cm) mobile animals were observed on each small artificial reef. The dominant fish species present was the gray snapper (Lutjanus griseus Linnaeus, 1758). Four yrs after the establishment of the artificial reefs, microphytobenthos abundance was twice as high in reef plots (1.7 ± 0.1 (micra)g chl-a cm-2) compared to control plots (0.9 ± 0.1 (micra)g chl-a cm-2). The accumulation of periphyton on glass periphytometers was four times higher in artificial reef plots (200.1 ± 45.8 mg chl-a m-2) compared to control plots (54.8 ± 6.8 mg chl-a m-2). The seagrass beds surrounding the artificial reefs changed rapidly, from a sparse Thalassia testudinum (Banks & Soland. ex Konig) dominated community, which persisted at control plots, to a community dominated by Halodule wrightii (Ascherson). Such changes mirror the changes induced in experimentally fertilized seagrass beds in Florida, strongly suggesting that the aggregations of animals attracted by artificial reefs concentrated nutrients in this oligotrophic seascape, favoring the growth of fast-growing primary producers like microphytobenthos and periphyton, and changing the competitively dominant seagrass from slow-growing T. testudinum to faster-growing H. wrightii in the vicinity of the reefs.

Epiphyte loads on seagrasses and microphytobenthos abundance are not reliable indicators of nutrient availability in oligotrophic coastal ecosystems

Despite marked gradients in nutrient availability that control the abundance and species composition of seagrasses in south Florida, and the importance of nutrient availability in controlling abundance and composition of epiphytes on seagrasses in other locations, we did not find that epiphyte load on the dominant seagrass, Thalassia testudinum, or that the relative contribution of algal epiphytes to the epiphyte community, was positively correlated with nutrient availability in the water column or the sediment in oligotrophic seagrass beds. Further, the abundance of microphytobenthos, as indicated by Chlorophyll- a concentration in the sediments, was not directly correlated with concentrations of nutrients in the sediments. Our results suggest that epiphyte and microphytobenthos abundance are not unambiguous indicators of nutrient availability in relatively pristine seagrass environments, and therefore would make poor candidates for indicators of the status and trends of seagrass ecosystems in relatively low-nutrient environments like the Florida Keys.

Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension

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 358:125-136 (2008) - DOI: https://doi.org/10.3354/meps07338 Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension John Widdows1,*, Nick D. Pope1, Mary D. Brinsley1, Harald Asmus2, Ragnhild M. Asmus2 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, Germany *Email: j.widdows@pml.ac.uk ABSTRACT: The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ0). Despite this increase in τ0 there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τe] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z. marina having little effect on sediment erodability (bare: τe = 0.05 Pa; Z. marina: τe = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment. KEY WORDS: Seagrass beds · Zostera noltii · Zostera marina · Hydrodynamics · Sediment resuspension Full text in pdf format PreviousNextCite this article as: Widdows J, Pope ND, Brinsley MD, Asmus H, Asmus RM (2008) Effects of seagrass beds (Zostera noltii and Z. marina) on near-bed hydrodynamics and sediment resuspension. Mar Ecol Prog Ser 358:125-136. https://doi.org/10.3354/meps07338 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 358. Online publication date: April 21, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Microalgae, macrofauna and sediment stability: an experimental test of a reciprocal relationship

A manipulative field experiment was conducted at Blackness in the Firth of Forth, Scotland, to study the effects of differences in microphytobenthos abundance on sediment stability and macrofauna. Opaque and transparent Perspex was used to construct shaded and controlled treatments, respectively. Shading resulted in significantly lower levels of chlorophyll a recorded from the sediment. Shaded treatments showed significant reductions in populations of Macoma balthica, Hydrobia ulvae and Corophium volutator, 3 of the dominant species at this site. Largely as a result of changes in the abundance of these species, multivariate analyses showed significant differences between shaded and control communities. The impacts of shading on individual species depended on the sediment characteristics. There were no significant differences between treatments in recorded sediment accretion/erosion rates. These results demonstrate the importance of microphytobenthos for the deposit feeders M. balthica, H. ulvae and C. volutator. The absence of any effect on sediment stability might result from the reciprocal, opposite effects of macrofauna and microphytobenthos on sediment stability cancelling each other out in this experiment.

Effects of the invertebrate infauna on early saltmarsh plant colonisation of managed realignment areas in south-east England

Saltmarsh erosion on the subsiding coastline of SE England is a major conservation problem and necessitates increased expenditure on maintaining sea walls. The preferred manage- ment solution is selective managed realignment (set-back), where sea walls are breached to allow new intertidal land to develop into saltmarsh. We tested the hypothesis that saltmarsh would not nec- essarily develop in low-lying realignment sites on subsiding coasts, because the accreting sediment would be colonised first by infaunal invertebrates, particularly the polychaete Nereis diversicolor, which would prevent development of vegetation through bioturbation and herbivory. The hypothe- sis was supported by data on the distributions of the plants and invertebrates in the experimental managed realignment sites at Tollesbury and Orplands, and from infauna exclusion experiments. Tollesbury mostly developed into mudflat colonised mainly by N. diversicolor and Hydrobia ulvae, with only a narrow zone of saltmarsh vegetation in the highest area where little sediment had accreted. In contrast, most of the Orplands site was relatively high and, apart from some low-lying basins, little sediment accretion occurred and the area was soon largely covered with saltmarsh vegetation. In experimental N. diversicolor exclusion areas, the abundance of microphytobenthos and filamentous algae and sediment accretion increased at both sites, but saltmarsh vegetation only developed at Orplands, probably because the seeds at Tollesbury were buried by rapidly accreting sediment. Further management of low-lying realignment areas is necessary to prevent invertebrate colonisation and help promote saltmarsh development. The experiments also indicate that marsh regeneration need not depend on realignment and should be possible on existing mudflats.

Selective feeding of littoral harpacticoids on diatom algae: hungry gourmands?

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm2, so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.