Soil-plant interactions in a neotropical mangrove forest: iron, phosphorus and sulfur dynamics

We examined soil porewater concentrations of sulfate, alkalinity, phosphorus, nitrogen, and dissolved organic carbon and solid phase concentrations of pyrite in relation to mangrove species distributions along a 3.1-km-long transect that traversed a 47.1-km² mangrove forest in the Dominican Republic. Iron, phosphorus, and sulfur dynamics are closely coupled to the activity of sulfate-reducing bacteria, the primary decomposers in anoxic soils of mangrove ecosystems. Patterns in the chemistry data suggested that sulfate reduction rates and storage of reduced sulfur were greater in the inland basin forest dominated by Laguncularia racemosa than the Rhizophora mangle dominated forest of the lower tidal region. The distribution of Laguncularia was significantly correlated with concentrations of total phosphorus (r= 0.99) and dissolved organic carbon (r= 0.86), alkalinity (r= 0.60), and the extent of sulfate depletion (r= 0.77) in the soil porewater and soil pyrite concentrations (r= 0.72) across the tidal gradient. Leaf tissue chemistry of Laguncularia was characterized by lower C:N and C:P ratios that could fuel the higher rates of decomposition in the Laguncularia-dominated forest. We suggest that a plant-soil-microbial feedback contributes to the spatial patterning of vegetation and soil variables across the intertidal zone of many mangrove forest communities. Received: 28 May 1997 / Accepted: 23 January 1998     

Recognizing and interpreting vegetational belts: New wine in the old bottles of a von Humboldt's legacy

Since von Humboldt, recognizing and using elevational subdivisions is at the core of biogeographical and ecological studies in mountain ecosystems. However, despite the large use of vegetational belts, their conceptual definition and practical identification appear to be surprisingly loose and inconsistent. Many authors use variations in climatic conditions to identify elevational belts. These belts are useful to set a framework for ecological studies but cannot be considered a surrogate of vegetational belts, because factors different from climate play a major role in determining the distribution of plant assemblages. Vegetation physiognomy can be used to identify ‘biome‐type’ belts that are useful for comparisons across geographical areas with different floras. However, to properly reflect ecological conditions at local scale, vegetational belts should be based on species composition. One of the most effective statistical approaches for this purpose is the use spatially constrained cluster analysis. The use of indicator species analysis may be also recommended to identify the species that most characterize vegetational belts. This can help researchers to identify belts in the field. Since species identification can be difficult, some authors use plant functional types for belt delimitation. Plant functional types can be helpful to trace the adaptative responses of vegetation along elevational gradients, but cannot be recommended as a standard way to identify belts. In general, criteria to identify vegetational belts can be based on both vegetation structure (namely physiognomy and structural parameters) and/or species composition, depending on the scale and the aim of the analyses, and they should be clearly stated.     

A methodology for screening haemolymph of intertidal mussels, Mytilus edulis, using FT-IR spectroscopy as a tool for environmental assesment

Developing effective, rapid and inexpensive methods for monitoring and conserving aquatic resources is an important issue for environmental managers. This study focuses on Mytilus edulis, a keystone species of many coastal marine communities, which is frequently used as a biomonitor for a range of pollutants. Recent advances in post-genomic technologies have provided new methods of biochemical screening, and Fourier transform-infrared spectroscopy (FT-IR) is one such method that could enable bioindicator species to be used for environmental assessment. This paper develops a methodology to apply the FT-IR approach to marine intertidal M. edulis and addresses three methodological issues: First, the optimum physical location for biofluid sampling is examined (i.e. laboratory versus field). Secondly, the effects of transportation of frozen biofluid sampling from either the field-site or laboratory to the analytical facility are considered. Finally, the effect of repeated FT-IR measurements on collected M. edulis haemolymph samples is examined. From these results we suggest sampling haemolymph from M. edulis at the top of the shore prior to immediate snap-freezing in liquid nitrogen. Sample transportation can occur on ice for up to eight hours before storage at −80 °C. FT-IR measurements should occur within three months of collection and samples should not be used or thawed more than twice. We show how this method can be used to differentiate successfully between four different estuarine environments. Ultimately, through addressing these methodological questions, we provide a protocol to allow efficient sampling and FT-IR measurement of M. edulis as collected from the intertidal areas of rocky and muddy shores. We conclude that due to current monitoring needs presented by the European Water Framework Directive such an approach could prove to be an invaluable future tool for assessing coastal water quality.     

Sources of organic matter for intertidal consumers on Ascophyllum-shores (SW Iceland): a multi-stable isotope approach

Stable isotopes were used to examine the origin of organic matter in Icelandic Ascophyllum-based habitats, the role of different organic matters in filling intertidal food webs and the food preferences of the most abundant suspension feeders, grazers and predators. We selected three intertidal sites on the SW coast of Iceland where we sampled in early September 2004, organic matter sources (POM, SOM and most abundant primary producers, A. nodosum and F. vesciculosus) and the most abundant macrofauna species (barnacles, mussels, gastropods, sponge and crabs). Even though the primary production (Ascophyllum-based) was the same at the three study sites, the isotopic composition of common-among-sites organisms varied due to local differences in the origin of available POM and SOM and in food web structures.     

ANALYSIS OF SPATIAL AND TEMPORAL DIVERSITY AND DISTRIBUTION OF PORPHYRA (RHODOPHYTA) IN SOUTHEASTERN NEW ZEALAND SUPPORTED BY THE USE OF MOLECULAR TOOLS1

Molecular studies have shown that New Zealand’s rocky shores are a habitat for >30 species of Porphyra, but little is known of their seasonal and zonal distribution. The spatial and temporal distribution of bladed Porphyra gametophytes at Brighton Beach, southeast New Zealand, were monitored for 32 months. Molecular markers were used for species identification, and a total of nine species was observed as being present during this time. Two species, P. cinnamomea and Porphyra sp. “ROS 54,” were the most common, and both were present for most months, while the remaining seven species were present sporadically, for only a few weeks at a time. P. cinnamomea W. A. Nelson and Porphyra sp. “ROS 54” were most common in the midintertidal, and both showed a similar seasonality with the highest presence during spring. They also showed a similar trend of seasonal dieback resulting in at least 1 month (May) in two consecutive years when they were both absent. This is one of the few studies investigating spatial and temporal distribution within a genus and over a 3‐year period. Our results show no distinct intertidal zonation patterns within the genus, and we conclude that morphologically similar species in a similar habitat rely on physiological mechanisms for survival.     

Small-scale burial of macroalgal detritus in marine sediments: Effects of Ulva spp. on the spatial distribution of macrofauna assemblages

Patches of dead seaweeds can deposit, bury, and age into the sediment. Decomposition and release of algal-derived nutrients can influence patterns of distribution of benthic organisms. Here, I investigated how small-scale burial of Ulva spp. affected spatial variation of macrofauna in intertidal sediment. I deliberately buried Ulva detritus under the surface of 50 × 50 cm² patches of sediment in three intertidal flats of the Oosterschelde estuary (The Netherlands). Results showed that there was no accumulation of particulate organic carbon and nitrogen in the sediment at the scales examined. The biomass of microphytobenthos did not show any change and there was evidence that grazing was important all over the study area. Burial did not alter composition and diversity of macrofauna, but some animals (Corophium volutator, Eteone spp. and Scoloplos armiger) had less numbers in the plots where detritus was buried than in the controls. These findings showed that burial of macroalgal detritus does not represent a major source of variation at the scales examined. It is suggested that in these sediments, recycling of detritus is fast and it buffers the effects of excess organic matter in the system.     

A comparison of statistical approaches to analyzing community convergence between natural and constructed oyster reefs

Evaluating the success of habitat creation or restoration depends primarily on the selection of appropriate goals, relevant metrics and robust analytic approaches. For intertidal oyster reefs, the goal of restoring ecological function often is as important as the production of harvestable oysters, especially since oysters are the habitat. Assessing differences in resident faunal composition between created and natural reefs is one possible metric for evaluating ecological success. Yearly changes in the resident faunal composition on constructed and natural intertidal oyster reefs at one South Carolina restoration site were analyzed with a variety of statistical approaches to determine the most effective method(s) for documenting possible convergence in the similarity of reef assemblages over time. Two datasets were defined by the level of taxonomic identification, all taxa or a subset of common taxa, and the level of taxa reduction; all taxa, taxa > 1% of total abundance, and taxa significantly contributing to variation. Data were analyzed using “classic” multivariate analysis of variance (MANOVA), null model analysis of co-occurrence (ECOSIM), nonparametric analysis of similarity (ANOSIM), and permutation tests for multivariate analysis of similarity (PERMANOVA). Taxa abundance was used to weight MANOVA and ECOSIM analyses, while the Bray-Curtis dissimilarity index was used to weight ANOSIM and PERMANOVA analyses. Initial constraints on the analytic design and data manipulations resulted in only one test where convergence of the constructed and natural reef assemblages was indicated. Prescribed reductions in the suite of taxa considered did not alter appreciably the results. The analytic approaches varied in suitability and effectiveness at discriminating among changes in compositional similarity, even when initial constraints were relaxed. MANOVA results indicated either no difference or a significant difference in resident faunal composition between reefs, but were compromised by the inability to transform the data sufficiently to test for multivariate homogeneity violations even in analyses with reduced taxa numbers. Interpretation of ECOSIM results suggested fewer taxa in common even on natural reefs and were affected by a lack of design alternatives and the possible inflation of Type I error that weighting by abundance may cause. ANOSIM results identified no significant reef treatment effects and also suffered from design constraints and an inability to generate enough permutations to test for significant differences in datasets with relatively small sample sizes. All test results from PERMANOVA analyses except one indicated unambiguously that resident faunal assemblages on constructed reefs generally were not yet similar to natural reefs even after 7 years. Convergence of constructed and natural reef resident assemblages was suggested by PERMANOVA tests only for the dataset with the fewest taxa. The negligible limitations of PERMANOVA, flexible design options, and ability to generate significance tests for small sample sizes make the approach powerful. The ongoing development of effective statistical approaches for testing the significance of taxonomic compositional changes among habitats makes the determination of whether restoration projects are successful less dependent on the choice of analytic technique. More critical, biological questions including whether convergence of taxa abundance and composition is a valid indicator of similar ecological function remain to be answered.     

Exploiting beach filling as an unaffordable experiment: Benthic intertidal impacts propagating upwards to shorebirds

Cold-season filling using much coarser sediments than the native caused dramatic suppression of beach macroinvertebrates, demonstrably degrading habitat value for foraging shorebirds. As a dual consequence of persistent steepening of the foreshore, which translated to reduction in habitat area by 14-29%, and disturbance-induced depression of invertebrate densities on filled beaches, abundances of Donax spp. and haustoriid amphipods averaged less than 10% of control levels. Donax spp. is the biomass dominant and a key prey for higher trophic levels. Haustoriids lack pelagic larvae. Recovery on filled beaches was not initiated by either taxon during the March-November sampling. Emerita talpoida, an order of magnitude less abundant than Donax spp. on control beaches, exhibited a pattern of initial depression on filled beaches but recovered by mid summer. Polychaetes, mostly the small Scolelepis squamata, experienced a warm-season bloom of equal magnitude on filled and control beaches. Summertime recruitment of predatory ghost crabs appeared inhibited on filled beaches, perhaps by persistent shell hash. Intertidal shell cover on filled beaches averaged 25-50% in mid summer as compared to 6-8% on control beaches. Largely in response to prey depression, but perhaps also to surface shell armoring and/or coarsening of sediments, shorebird (mostly sanderling) use plummeted by 70-90% on filled beaches until November. Thus, despite likely adaptations to natural sediment dynamics, the high intensity of sediment deposition, cumulative spatial scope (10.8 km), and unnaturally coarse shelly character of the Bogue Banks beach nourishment resulted in a perturbation that exceeded biotic resistance and degraded the trophic transfer function of this highly productive habitat for at least one warm season.     

In situ measurements of hydrodynamic forces imposed on Chondrus crispus Stackhouse

Among the hydrodynamic forces experienced by intertidal organisms, drag and the impingement force are thought to have the greatest effect on macroalgae. These forces are modified by biotic factors such as algal morphology, reconfiguration, and the presence of a canopy. However, much of what is known about the hydrodynamics of macroalgae has been garnered from low-velocity laboratory flume studies. Few field studies have measured drag and none have directly measured the effects of the canopy on force. To examine in situ hydrodynamic forces imposed on the turf forming macroalga Chondrus crispus, compact digital force sensors were developed that measure and record the 3-dimensional force imposed on a macroalga without disturbing the surrounding canopy. Sensors were positioned within natural Chondrus beds and the effects of the canopy, algal morphology, and sea state on in situ hydrodynamic force were examined. Additionally, the predictions of a new model for drag on flexible macroalgae were tested by simultaneously measuring force and water velocity. Digital force recordings indicated that Chondrus only experience drag; lift and impingement force were negligible in all combinations of factors. Canopies significantly reduced drag by 15-65%. Morphology and size also influenced drag, such that lower forces were imposed on small planar algae than large arborescent individuals. Further, planar algae experienced low drag in all combinations of sea and canopy state, indicating that these individuals may not be as susceptible to wave disturbance as arborescent individuals. Overall, these data indicate that the ability for Chondrus to grow large, arborescent individuals is dependent on the drag reducing properties of the canopy, while more hydrodynamically harsh habitats may be accessible to planar morphologies. Additionally, these data suggest that drag models for canopy forming macroalgae must incorporate the effects of the canopy to predict drag accurately in situ.     

Larval Chironomid Community Organization in Four Tropical Rivers: Human Impacts and Longitudinal Zonation

Chironomid (Diptera) larvae were collected between February 1997 and December 1998 from four Trinidadian rivers, two relatively impacted (the Arima and Guanapo Rivers), and two comparatively pristine (the Marianne and Paria Rivers), in order to assess community changes as a result of both land-use and the natural longitudinal gradient. Multivariate analyses indicated changes in chironomid community composition at the impacted sites, with the most dramatic changes occurring in the low-reaches of the Arima and Guanapo Rivers, which were subjected to nutrient enrichment and general urban runoff. For example, genus richness, measured using Marglef’s Index, was lower in the low-reaches of both the Arima and Guanapo Rivers (0.9 and 1.1, respectively), compared with the low-reaches of the Marianne and Paria Rivers (1.8 and 2.8, respectively). The average chironomid densities in the low-reaches of both the Arima and Guanapo Rivers (1346 and 1029 chironomids/1 m², respectively) were also much higher compared with those of the Marianne and Paria Rivers (130 and 123 chironomids/1 m², respectively), most likely being the result of nutrient enrichment. Furthermore, the low-reaches of both the Arima and Guanapo Rivers demonstrated a shift in community structure, favouring those taxa more tolerant of non-point source pollution, including Thienemanniella spp., Paratrichocladius sp., Corynoneura spp., Polypedilum spp., Chironomus spp., Saetheria sp., Rheotanytarsus sp., and Thienemannimyia sp. In terms of natural longitudinal changes, the chironomid community shifted from one numerically dominated by Orthocladiinae in the headwaters to one numerically dominated by Chironominae in the low-reaches. Overall, this study shows that land-use can have a greater influence on the structure and diversity of chironomid communities than the natural longitudinal gradient.