Biotic Disturbance,Recolonization, and Early Succession of Bacterial Assemblages in Intertidal Sediments

The role of disturbance in structuring natural microbial communities has been largely unexplored. Disturbance associated with invertebrate ingestion can reduce bacterial biomass and alter metabolic activities and compositions of bacterial assemblages in marine sediments. The primary objectives of the research presented here were to test whether ingestion by a taxonomically diverse group of deposit feeders constituted a disturbance, and to determine the mechanisms by which bacterial assemblages recover following deposit-feeder ingestion. To test the question of disturbance, we compared fresh egesta vs surficial sediments with respect to bacterial assemblage structure. In emersed intertidal sediments, microbial recovery could be due to regrowth of bacterial populations surviving gut passage or to immigration from adjacent sediments. To differentiate between these modes of recolonization we used field manipulative experiments to exclude migration by isolating freshly extruded fecal coils of three deposit-feeding species from surrounding sediments. We then followed the quantitative and qualitative recovery in egesta and sediments through time using epifluorescence microscopy and PCR-DGGE analysis of 16S rDNA. Our findings indicate that (1) the degree and nature of the disturbance to bacterial assemblages from deposit feeding varies among invertebrate taxa, (2) recovery was significant but incomplete over 3 h, and (3) recolonization of biotically disturbed sediments is dominated by immigration.This revised version was published online in November 2004 with corrections to Volume 48.     

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.     

Pathways for algal recolonization in seasonally‐flowing streams

1. In semi‐arid climates, seasonally‐flowing streams provide most of the water required for human use, but knowledge of how water extraction affects ecological processes is limited. Predicted alterations in stream flows associated with the impacts of climate change further emphasize the need to understand these processes. Benthic algae are an important base for stream food webs, but we have little knowledge of how algae survive dry periods or respond to altered flow regimes. 2. We sampled 19 streams within the Grampians National Park, south‐eastern Australia and included four components: a survey of different drought refuges (e.g. permanent pools, dry biofilm on stones and dry leaf packs) and associated algal taxa; a survey of algal regrowth on stones after flows recommenced to determine which refuges contributed to regrowth; reciprocal transplant experiments to determine the relative importance of algal drift and regrowth from dry biofilm in recolonization; direct measurement of algal drift to determine taxonomic composition in relation to benthic assemblage composition. 3. Algae showed little specificity for drought refuges but did depend on them; no species were found that were not present in at least one of the perennial pool, dry biofilm or leaf pack refuges. Perennial pools were most closely correlated with the composition of algal assemblages once flows resumed, but the loss or gain of perennial pools that might arise from stream regulation is unlikely to affect the composition of algal regrowth. However, regulated streams were associated with strong increases in algal density in dry biofilm, including increased densities of Cyanobacteria. 4. A model for algal recolonization in seasonally‐flowing streams identified three pathways for algal recolonization (drift‐dependent, dry biofilm‐dependent and contributions from both), depending on whether streams are diatom‐dominated or dominated by filamentous algae. The model predicted the effects of changes to stream flow regimes on benthic algal recolonization and provides a basis for hypotheses testable in streams elsewhere.     

Spatial and halophyte-associated microbial communities in intertidal coastal region of India

Microbial communities in intertidal coastal soils respond to a variety of environmental factors related to resources availability, habitat characteristics, and vegetation. These intertidal soils of India are dominated with Salicornia brachiata, Aeluropus lagopoides, and Suaeda maritima halophytes, which play a significant role in carbon sequestration, nutrient cycling, and improving microenvironment. However, the relative contribution of edaphic factors, halophytes, rhizosphere, and bulk sediments on microbial community composition is poorly understood in the intertidal sediments. Here, we sampled rhizosphere and bulk sediments of three dominant halophytes (Salicornia, Aeluropus, and Suaeda) from five geographical locations of intertidal region of Gujarat, India. Sediment microbial community structure was characterized using phospholipid fatty acid (PLFA) profiling. Microbial biomass was significantly influenced by the pH, electrical conductivity, organic carbon, nitrogen, and sodium and potassium concentrations. Multivariate analysis of PLFA profiles had significantly separated the sediment microbial community composition of regional sampling sites, halophytes, rhizosphere, and bulk sediments. Sediments from Suaeda plants were characterized by higher abundance of PLFA biomarkers of Gram-negative, total bacteria, and actinomycetes than other halophytes. Significantly highest abundance of Gram-positive and fungal PLFAs was observed in sediments of Aeluropus and Salicornia, respectively than in those of Suaeda. The rhizospheric sediment had significantly higher abundance of Gram-negative and fungal PLFAs biomarkers compared to bulk sediment. The results of the present study contribute to our understanding of the relative importance of different edaphic and spatial factors and halophyte vegetation on sediment microbial community of intertidal sediments of coastal ecosystem.     

Recovery of plankton from hurricane impacts in a large shallow lake

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Lack of recovery from disturbance in high-arctic boulder communities

High-arctic boulder communities that are impacted by anthropogenic and natural influences can result in the removal or scouring of sessile organisms leaving either open space or damaged organisms. This project asks how sessile communities recover after disturbances by determining (1) timing of recolonization, (2) grazer effects on recolonization, and (3) vegetative regrowth rates for encrusting sponges and corallines. Cleared boulders were monitored over 7 years to determine recolonization timing and grazer impacts. Vegetative regrowth rates were determined by monitoring partial clearings for 4 years. This study found that recolonization was slow with less than 10 % of the boulder surfaces being colonized after 7 years. Recolonization was so slow that it was difficult to ascertain grazer impacts, although it appeared there were no impacts. Lastly, this study showed that vegetative regrowth of sponges and encrusting coralline algae was fast. Sponges averaged 100 % regrowth after 2 years and corallines averaged 40 % after 4 years. Coralline regrowth was slower when exposed to higher sedimentation. This study showed that community recovery from disturbances is very slow in the Beaufort Sea, especially if entire organisms are removed. However, if removal is partial, recovery can be quicker, particularly in low sediment areas. Complete community recovery in this system after a disturbance may take a decade or more.     

Palaeoenvironment and taphonomy of a Late Jurassic (Late Tithonian) Lagerstätte from central Poland

A rich assemblage of exceptionally preserved marine and terrestrial fossils occurs in fine‐grained limestones in the upper part of the Late Tithonian (Middle Volgian) shallowing upward carbonate sequence in Central Poland. The richest horizon, a deposit known locally as the Corbulomima horizon, is named after the shallow burrowing suspension feeding bivalve Corbulomima obscura, moulds of which occur in densities of up to 500 per square metre on some bedding planes. The fauna in this bed also includes organic and phosphatic remains of a wide range of other creatures including the exuviae of limulids and decapods, disarticulated fish skeletons and rare isolated pterosaur bones and teeth. There are also perfectly preserved dragonfly wings and beetle exoskeletons. The average stable carbon and oxygen isotope values for ostracod shells and fine‐grained sediment from this horizon suggest precipitation of the calcium carbonate from warm seawater of normal marine salinity. The carbonate sediments overlying the fossiliferous horizon have been interpreted as nearshore to shoreface facies. These pass abruptly into coarse reworked intraclastic sediments interpreted as possible tsunami or storm surge over‐wash deposits. The clasts in this deposit have more positive oxygen isotope values than those in the underlying limestone, which may indicate that they were lithified in a slightly more evaporative, perhaps intertidal, setting. The succession terminates with silicified fine‐grained limestones likely to have formed in extremely shallow lagoonal environments. In contrast with the Solnhofen limestones of Lower Tithonian age in south‐central Germany the Corbulomima horizon is interpreted as a transitional deposit formed in a shallow marine setting by rapid burial with elements of both Konservat‐ and Konzentrat‐Lagerstätte preservation. □Konzentrat and Konservat‐Lagerstätte, Taphonomy, Palaeoenvironment, Paleogeography, Late Jurassic, Poland.     

Secondary forest growth deviation from chronosequence predictions in central Amazonia

Nearly all published rates of secondary forest (SF) regrowth for Amazonia are inferred from chronosequences. We examined SF regrowth on abandoned pastures over a 4‐year period to determine if measured rates of forest recovery differ from chronosequence predictions. We studied the emergence, development and death of over 1300 stems in 10 SFs representing three age classes (<1–5, 6–10 and 11–14 years old). Mean tree biomass accumulation in both the <1–5 and 6–10 years old (4.4 and 5.7 Mg ha⁻¹ yr⁻¹, respectively) abandoned pastures was lower than predicted and deviated significantly (57% and 41%) from rates estimated from the chronosequence. The older SFs, with a mean growth rate of 9.9 Mg ha⁻¹ yr⁻¹ followed the rate predicted by the chronosequence. Understocking was the primary cause of low biomass recovery rates in the youngest forests; although the youngest stands had a diameter at breast height increment three times the oldest stands, the youngest stands lacked sufficient density to cumulatively produce high biomass accumulation rates. Four years of measurement indicated that the youngest stands had developed 59% of the stems measured in the older stands during the same time period. The 6–10‐year‐old stands were rapidly self‐thinning and approached stem density values measured in the same aged stands at the onset of the study. Mortality was high for all stands, with 54% of the original stems remaining after 4 years in intermediate‐aged stands. The forests were dominated by the tree Vismia, which represented 55–66% of the biomass in all stands. The Vismia share of the biomass was decreasing over time, with other genera replacing the pioneer. Our measured rates of regrowth indicate that generalized estimates of forest regrowth through chronosequence studies will overestimate forest regrowth for the youngest forests that were under land use for longer time‐periods before abandonment. Certified Emission Reductions under the Clean Development Mechanism of the Kyoto protocol should consider these results when predicting and compensating for carbon sequestered under natural forest management.     

Pyrite formation in marshes during early diagenesis

Pyrite was removed from peat cores by draining the sediments and allowing the pyrite to oxidize. Then the peat cores were placed back into intertidal salt marsh sediments to incubate. Pyrite accumulated rapidly in peat incubated in situ. A greater accumulation of pyrite was observed in peat that contained living grass than peat in which the grass had been killed.

Resin‐imbedded samples of peat from nearby sediments showed that small single crystals of pyrite were abundant, supporting the idea that pyrite in marshes forms rapidly through direct precipitation. Pyrite was also observed filling vascular channels in roots. It had been proposed that pyrite fills root channels in freshwater environments where the primary sulfur source used by sulfate‐reducing bacteria is organic sulfur rather than sulfate. The widespread occurrence of pyrite filling vascular channels in salt marsh peat makes it unlikely that pyrite morphology can be used to infer the salinity of the overlying water.

Marsh sediments are characterized by higher carbon/sulfur ratios and pyritization (Fe‐pyritel(Fe‐pyrite + Fe‐HCl)) indices than marine subtidal sediments. Within wide ranges these indices do not seem to be very sensitive to salinity of flooding water or carbon concentrations in sediments. Oxidation and iron availability appear to be the major controls on pyrite accumulation in marshes. While pyrite concentrations in submerged sediments can be used as indicators of relative rates of sulfate reduction, sulfur storage in intertidal marsh sediments is not as tightly linked to this microbial process.     

Community and population-level responses to disturbance in a sandflat community

The responses of soft sediment infauna were investigated in an intertidal sandflat to determine patterns of recolonization and succession at the community and population level. Experimental disturbance plots, 1 m², were initiated in August and sampled for 4.5 months along with ambient sediments. Sediment grain-size was used as a general indicator of the physical state of the disturbance patches, and grain-size distributions among disturbance and ambient patches became similar after ∼2.5 months. Recolonization varied among the dominant infaunal taxa. Densities of infauna that were most abundant in the habitat, primarily syllid polychaetes, did not recover to ambient levels until 3-4 months after disturbance, when ambient densities were falling to winter lows. Multivariate analysis indicated that community recovery occurred by the end of the study period after 4.5 months. Although community structure recovered by the end of the study, the population structure of the dominant species Parapionosyllis longicirrata remained significantly different among ambient and disturbed patches. On all sampling dates except one, disturbance patches had a higher number of larger individuals than ambient sediments. Previous studies have shown that bedload transport of juveniles and adults, and other processes, can cause recolonization to be relatively rapid on intertidal sandflats. However, our results indicate that recovery times may be on the order of months at large disturbance sizes. Therefore, rapid responses may occur primarily in the case of small-scale (<1 m²) disturbance patches. Secondly, recovery at the community level does not necessarily mean that population-level characteristics of species comprising the community have recovered. Population-level differences may be longer lasting than indicated by community level indicators of recovery.     

Geochemical composition of faecal pellets as an indicator of deposit‐feeding strategies in the trace fossil Phymatoderma

Phymatoderma is a branching burrow system consisting of tunnels filled with faecal pellets, and it has been interpreted as a product of a surface deposit‐feeding animal. Elemental analyses of Phymatoderma were conducted to reveal the specific feeding mode of its tracemaker, using samples from the Lower Jurassic epicontinental shelf deposits in the Dotternhausen section, southern Germany and from the Upper Pliocene continental slope deposits in the Shioura section, central Japan. Elemental compositions of the pelletal infill of Phymatoderma and its overlying mudstone from the Dotternhausen section show no significant difference, suggesting that the tracemaker was a non‐selective deposit feeder. In contrast, elemental compositions of the tuffaceous pellets of Phymatoderma from the Shioura section and its overlying volcanic ash show a difference in trends: CaO is significantly concentrated in the pellets. Because microfossils such as foraminifera and coccoliths are occasionally found in the tuffaceous pellets, CaO accumulation in the pelletal infill indicates the Phymatoderma‐producer that lived in the Pliocene slope setting selectively ingested particles with higher biomass of such microorganisms (or ingested microorganisms themselves) when feeding the surface sediments. Although two types of feeding modes of the tracemaker were recognized in Phymatoderma between different bathymetrical settings, each feeding mode seems to be an effective strategy to intake nutrients from the surface sediments and to reflect an adaptation of the tracemaker to the food contents in the surrounding substrate. This study suggests that geochemical composition of faecal pellets of trace fossils can be a useful indicator of grain‐selective/non‐selective deposit‐feeding strategies of ancient animals.     

Effects of shelter and enrichment on the ecology and nutrient cycling of microbial communities of subtidal carbonate sediments

The interactions between physical disturbances and biogeochemical cycling are fundamental to ecology. The benthic microbial community controls the major pathway of nutrient recycling in most shallow-water ecosystems. This community is strongly influenced by physical forcing and nutrient inputs. Our study tests the hypotheses that benthic microbial communities respond to shelter and enrichment with (1) increased biomass, (2) change in community composition and (3) increased uptake of inorganic nutrients from the water column. Replicate in situ plots were sheltered from physical disturbance and enriched with inorganic nutrients or left without additional nutrients. At t(0) and after 10?days, sediment-water fluxes of nutrients, O(2) and N(2) , were measured, the community was characterized with biomarkers. Autochthonous benthic microalgal (BMA) biomass increased 30% with shelter and a natural fivefold increase in nutrient concentration; biomass did not increase with greater enrichment. Diatoms remained the dominant taxon of BMA, suggesting that the sediments were not N or Si limited. Bacteria and other heterotrophic organisms increased with enrichment and shelter. Daily exchanges of inorganic nutrients between sediments and the water column did not change in response to shelter or nutrient enrichment. In these sediments, physical disturbance, perhaps in conjunction with nutrient enrichment, was the primary determinant of microbial biomass.     

Reduction and recovery of keystone predation pressure after disease‐related mass mortality

Disturbances such as disease can reshape communities through interruption of ecological interactions. Changes to population demographics alter how effectively a species performs its ecological role. While a population may recover in density, this may not translate to recovery of ecological function. In 2013, a sea star wasting syndrome outbreak caused mass mortality of the keystone predator Pisaster ochraceus on the North American Pacific coast. We analyzed sea star counts, biomass, size distributions, and recruitment from long‐term intertidal monitoring sites from San Diego to Alaska to assess regional trends in sea star recovery following the outbreak. Recruitment, an indicator of population recovery, has been spatially patchy and varied within and among regions of the coast. Despite sea star counts approaching predisease numbers, sea star biomass, a measure of predation potential on the mussel Mytilus californianus, has remained low. This indicates that post‐outbreak populations have not regained their full predation pressure. The regional variability in percent of recovering sites suggested differences in factors promoting sea star recovery between regions but did not show consistent patterns in postoutbreak recruitment on a coast‐wide scale. These results shape predictions of where changes in community composition are likely to occur in years following the disease outbreak and provide insight into how populations of keystone species resume their ecological roles following mortality‐inducing disturbances.     

Monitoring of physically restored seagrass meadows reveals a slow rate of recovery for Thalassia testudinum

Physical damage by motor vessels is a widespread problem for seagrass meadows, with hull and propeller strikes accounting for thousands of acres of impaired habitat in Florida, United States, alone. Because the excavations can become topographically unstable, and because recolonization and succession of seagrasses can require decades to reach climax, Thalassia testudinum‐dominated communities, there has been increasing efforts to regrade and stabilize impacted sediments, and to speed succession. A prior project involving eight vessel groundings in two hydrodynamic settings (high and low energy) examined the relative efficacy of capping injuries with sand‐filled fabric tubes or limestone pea rock, followed by planting of fast‐growing seagrass species and nutrient amendment using bird‐roosting stake deployments. Monitoring after 4 years showed recruitment of fast‐growing, subordinate species: Syringodium filiforme or Halodule wrightii, particularly in low‐energy environments; however, T. testudinum had not yet returned to natural densities. The current study extended monitoring an additional 3 years. At 7 years posteffort, T. testudinum recolonization was still incomplete. Of the eight sites, only three had statistically recovered. In low‐energy areas, H. wrightii cover was greater than in reference meadows, and a strong inverse relationship between H. wrightii and T. testudinum was observed. One explanation is that residual nutrients from bird feces switched the competitive outcome. We demonstrate using seagrass tissue N:P that nutrients delivered via bird stakes remained in the sediments of low‐energy environments, and argue that prolonged fertilization resulted in competitive advantages for H. wrightii, depressing T. testudinum recruitment and delaying recovery of the targeted seagrass community.     

EFFECT OF SUBSURFACE NUTRIENT SUPPLIES ON THE VERTICAL MIGRATION OF EUGLENA PROXIMA (EUGLENOPHYTA)1

Euglena proxima Dangeard inhabits intertidal sand flats and displays a tidal rhythm in vertical migration. During daytime low tides when the sand flat is emersed, millions of cells are visible on the sediment surface, but the population remains below the surface at all other times. An earlier study demonstrated that the extent of downward migration of E. proxima is reinforced by the presence of a subsurface layer of black sediment. The present study was designed to test the hypothesis that the higher availability of inorganic nutrients or organic substrates in or above the black layer is responsible for the enhancement of downward migration in E. proxima. This hypothesis was tested experimentally by manipulating the bottom water in 24 mesocosm containers in a tidal tank. Six replicates of each of the following nutrient treatments were tested: seawater control; deep porewater collected from 70 cm below the sediment surface; seawater enriched with ammonium, nitrate, and phosphate; and seawater enriched with acetate, glucose, and the preceding inorganic nutrients. Multivariate analysis of variance revealed that the chl a biomass and chl a‐weighted mean depth of the population at high tide were significantly greater for replicates receiving inorganic nutrients. There was no difference between those receiving only inorganic nutrients and those enriched with inorganic nutrients, acetate, and glucose. These findings represent the first experimental evidence that subsurface nutrients are an important resource that reinforces the maintenance of vertical migration behavior in benthic microalgae.     

Eukaryotic diversity in late Pleistocene marine sediments around a shallow methane hydrate deposit in the Japan Sea

Marine sediments contain eukaryotic DNA deposited from overlying water columns. However, a large proportion of deposited eukaryotic DNA is aerobically biodegraded in shallow marine sediments. Cold seep sediments are often anaerobic near the sediment–water interface, so eukaryotic DNA in such sediments is expected to be preserved. We investigated deeply buried marine sediments in the Japan Sea, where a methane hydrate deposit is associated with cold seeps. Quantitative PCR analysis revealed the reproducible recovery of eukaryotic DNA in marine sediments at depths up to 31.0 m in the vicinity of the methane hydrate deposit. In contrast, the reproducible recovery of eukaryotic DNA was limited to a shallow depth (8.3 m) in marine sediments not adjacent to the methane hydrate deposit in the same area. Pyrosequencing of an 18S rRNA gene variable region generated 1,276–3,307 reads per sample, which was sufficient to cover the biodiversity based on rarefaction curves. Phylogenetic analysis revealed that most of the eukaryotic DNA originated from radiolarian genera of the class Chaunacanthida, which have SrSO₄ skeletons, the sea grass genus Zostera, and the seaweed genus Sargassum. Eukaryotic DNA originating from other planktonic fauna and land plants was also detected. Diatom sequences closely related to Thalassiosira spp., indicative of cold climates, were obtained from sediments deposited during the last glacial period (MIS‐2). Plant sequences of the genera Alnus, Micromonas, and Ulmus were found in sediments deposited during the warm interstadial period (MIS‐3). These results suggest the long‐term persistence of eukaryotic DNA from terrestrial and aquatic sources in marine sediments associated with cold seeps, and that the genetic information from eukaryotic DNA from deeply buried marine sediments associated with cold seeps can be used to reconstruct environments and ecosystems from the past.     

A Doubling of Microphytobenthos Biomass Coincides with a Tenfold Increase in Denitrifier and Total Bacterial Abundances in Intertidal Sediments of a Temperate Estuary

Surface sediments are important systems for the removal of anthropogenically derived inorganic nitrogen in estuaries. They are often characterized by the presence of a microphytobenthos (MPB) biofilm, which can impact bacterial communities in underlying sediments for example by secretion of extracellular polymeric substances (EPS) and competition for nutrients (including nitrogen). Pyrosequencing and qPCR was performed on two intertidal surface sediments of the Westerschelde estuary characterized by a two-fold difference in MPB biomass but no difference in MPB composition. Doubling of MPB biomass was accompanied by a disproportionately (ten-fold) increase in total bacterial abundances while, unexpectedly, no difference in general community structure was observed, despite significantly lower bacterial richness and distinct community membership, mostly for non-abundant taxa. Denitrifier abundances corresponded likewise while community structure, both for nirS and nirK denitrifiers, remained unchanged, suggesting that competition with diatoms for nitrate is negligible at concentrations in the investigated sediments (appr. 1 mg/l NO₃^-). This study indicates that MPB biomass increase has a general, significantly positive effect on total bacterial and denitrifier abundances, with stimulation or inhibition of specific bacterial groups that however do not result in a re-structured community.     

INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.     

A field experiment on the effect of two types of sediment disturbance on the rate of recovery of a meiobenthic community in a eutrophicated lagoon

A recolonization field experiment of two different artificially disturbed sediments (both defaunated sand and defaunated/reduced sand through organic enrichment) was carried out in the Sacca di Goro (Adriatic sea, Po river Delta, Italy). Copepods showed themselves better colonizers than nematodes. In particular, copepods, in the defaunated sand, were able to reach the same densities as the control site after only seven days from the beginning of the experiment. In the reduced-sand, copepod recolonization occurred more slowly but reached the densities found in both azoic and control sediments at the end of the experiment (15 days), when the values of total carbon content decreased. The recovery evolution of the community structures was mostly dependent on the different behaviour of the active epibenthic species of the harpacticoids (e.g. Canuella perplexa T & A. Scott, 1893, Ameira parvula (Claus, 1866), Robertgurneya similis (A. Scott, 1896)) and of the passively transported endobenthic ones (e.g. Asellopsis sarmatica Jakubisiak, 1938, Ectinosoma dentatum Steuer, 1940).     

Recolonization of infauna on a tidal flat: An experimental analysis of modes of dispersal

Animals can colonize intertidal sediments by lateral movement through (by burrowing) and on (by crawling) the sediment or by settling (vertically) from the water-column (during larval or bed-load transport above the surface of the sediment). To determine the relative importance of these in recolonization of cleared areas, animals were experimentally prevented from lateral movement through sediments (using fences) and/or by settlement from the water-column (using lids). Necessary controls required nine experimental treatments and an untouched control. After 16 days, assemblages in experimental treatments differed from natural assemblages: recovery was not yet complete. There were differences among assemblages in plots with lids, but no differences among assemblages in plots with fences. After 33 days, treatments with lids still showed little recolonization. Multivariate analyses of assemblage showed the relevance of colonization from the water-column. Analysis of individual taxa revealed complex patterns of recolonization. In general, juvenile bivalves used the water-column, but numbers of large polychaetes were reduced by the fences, indicating that lateral movement was important. These experiments illustrate the complex dynamics of soft-bottom benthic assemblages and great importance of small-scale dispersal.