Life on the edge: conspecific attraction and recruitment of populations to disturbed habitats

Variation in the recruitment of benthic marine invertebrates is often attributed to the interaction of the supply of new individuals to a habitat and the availability of space for colonisation when they arrive. Also important in determining variation in recruitment is the response of the larvae to the characteristics of the habitat. Larvae of many benthic marine invertebrates have shown great specificity of requirements in setting their limits of distribution at the time of selection of a habitat. The tubeworm Galeolaria caespitosa shows great variation in recruitment from place to place on rocky intertidal seashores and is a gregarious animal with larvae showing directed responses to conspecific adults on the substratum. I hypothesised that, if variation in recruitment of G. caespitosa were independent of conditions on the substratum, the magnitude of recruitment in patches of the same shape but different sizes cleared within continuous mats of conspecific adults would be directly related to the area available for colonisation in the patch. Alternatively, if variation in recruitment were due to the response of larvae to conspecific adults on the substratum, the magnitude of recruitment would be a function of the perimeter of the patch, which, given patches of the same shape, is a measure of the influence of conspecific adults in that patch. To distinguish between these alternatives, small (area = 25 cm²; perimeter = 20 cm) and large (area = 225 cm²; perimeter = 60 cm) square patches were cleared within continuous mats of conspecific adults at four sites and recruitment of G. caespitosa was monitored over two seasons of recruitment. The density of recruits per unit area was, on average, almost three times greater in small than in large patches, indicating that recruitment of G. caespitosa is not directly related to the area of the patch. In contrast, the density of recruits per unit perimeter was not significantly different between small and large patches, indicating that recruitment of G. caespitosa is related to the proximity of conspecific adults in the patch. Therefore, at a given site, the perimeter of patches within mats of G. caespitosa is a better predictor of the relative magnitude of recruitment among patches than that provided by their areas. These results are contrary to many models of invertebrate recruitment that assume close linkage between available space on the substratum and settlement. Moreover, they highlight the importance of behavioural responses of animals at the time of selection of habitat in accounting for variation in recruitment. For populations of organisms that display gregarious behaviour at settlement, or conspecific attraction, this direct relationship between the perimeter of patches and recruitment could be used as a tool in restoring populations to disturbed habitats. The added benefit of such facilitative interactions in restoring populations is that they provide increasing returns to the population for a given supply of potential colonists to a habitat. Received: 1 November 1996 / Accepted: 20 January 1997     

Shallow- and deep-water mollusc distribution at Terra Nova Bay (Ross Sea, Antarctica)

The aim of this work is to improve the knowledge of the mollusc fauna of Terra Nova Bay (Ross Sea), on the basis of more than 100 stations sampled from 25 to 1100 m depth, during Italian Antarctic Expeditions (austral summers 1987/1988, 1989/1990, 1993/1994, 1994/1995, 1995/1996 and 1997/1998). In shallow waters, gastropod fauna is represented by 31 species, among which Neobuccinum eatoni, Onoba gelida, Powellisetia deserta, Philine cf. apertissima and Austrodoris kerguelenensis are the most abundant, while among bivalves (25 species) the commonest species are Adamussium colbecki, Yoldia eightsi, Laternula elliptica and “Montacuta”nimrodiana. At present, most of the Antarctic mollusc species are known as having circumantarctic distribution and are considered eurybathic. Moreover, in general, only scarce data are available on their substrate preferences, because of their occurrence in scattered sampling stations with different sediment features. In the present study carried out at Terra Nova Bay, based on a relatively high number of stations, a clear zonation of mollusc assemblages is shown, according to depth and sediment features. While gastropods characterise the superficial algal belts dominated by the red algae Iridaea cordata and Phyllophora antarctica, bivalve distribution is wider. Adamussium colbecki is the dominant species in the upper 100 m, both on soft and hard bottoms, if the slope is suitable (density up to 40–60 ind./m²). On coarse sands, this species is frequently accompanied by L. elliptica (density <20 ind./m²), while Y. eightsi characterises organic- enriched bottoms (density 70–80 ind./m²). Below 200 m depth Adacnarca nitens, Limatula hodgsoni, “Montacuta”nimrodiana and Mysella gibbosa are commonly found on relatively coarse sediments, while Thyasira dearborni and Yoldiella ecaudata prefer muddy bottoms. In order to identify bivalve assemblages, multivariate analyses were applied to stations grouped into depth ranges, sediment features and location. While shallow bathymetric ranges (25–200 m) are clearly defined, deeper clusters of stations are more influenced by the high degree of eurybathy of most of the species and by the scattered distribution of the sediment particle sizes. Accepted: 18 September 1999     

Distribution of zooplankton in the estuary of Keretskaya Guba (White Sea)

The horizontal heterogeneity of common zooplankton species in the Keretskaya Guba tidal estuary in the White Sea was studied. Based on an analysis of a series of simultaneously taken samples and the dispersion index, I _w = σ²/[`(m)]\bar{m} (where σ<sup>2</sup> is dispersion and [`(m)]\bar{m} is average population number), the existence of two and more levels of aggregation in most of the studied zooplankton species was found. The formation of patches of 1–20 m was most likely determined by biological causes, while patches some hundreds of meters long were formed due to the hydrodynamic conditions of the area.     

Floating-leaved macrophyte (<Emphasis Type="Italic">Trapa quadrispinosa</Emphasis> Roxb) beds have significant effects on sediment resuspension in Lake Taihu,China

Trapa quadrispinosa Roxb is a common floating-leaved macrophyte in China. In this study, the effects of Trapa on sediment resuspension in Lake Taihu, a large, shallow, eutrophic lake in eastern China, were investigated using sediment traps. The study was conducted at stations with and without Trapa beds from 7 September to 18 October, 2003. Results showed that sediment resuspension rates at the station without Trapa were significantly higher than those at the station with Trapa. During the study period (41 days), 10,970 g dw m⁻² of surface sediment was resuspended within the Trapa beds and 29,903 g dw m⁻² in the pelagic zone. These equate to phosphorus fluxes of 7.4 g m⁻² to the water column at the station with Trapa and 16.1 g m⁻² at the station without Trapa. Floating-leaved macrophytes, such as Trapa, may be important in sediment resuspension and internal phosphorus loading in shallow lakes.     

Influence of submerged macrophytes on sediment composition and near-bed flow in lowland streams

1. Submerged macrophytes have important physical and structural effects on lowland streams. This study investigated the ability of submerged macrophytes to modify the near-bed flow and to retain mineral and organic particles in patches of four common macrophytes in shallow Danish streams during mid-summer. 2. In dense patches of Callitriche cophocarpa and Elodea canadensis, where near-bed velocity was reduced, the sediment surface was markedly raised and enriched with fine particles. In dense patches of Ranunculus peltatus, fine sediments were deposited among rooted shoots in the upstream part of the patches, while erosion and coarse sediments prevailed in the downstream part of the patches because of the strong vortices that formed at the rear and moved up under the trailing canopy. The open canopy of Sparganium emersum, with its streamlined leaves, had little effect on flow and sediment. 3. Patterns of sediment deposition and composition were closely related to the morphology and canopy structure of plant species and the presence of low velocity above the sediment among the rooted shoots. The mineral particles retained probably originate from bed-load, and the enrichment with finer particles within the patches probably results mainly from size-selective processes during erosion and transport of particles rather than during deposition. The mixed sediment composition within patches suggests that the flow-resistant shoots generate an environment conducive to deposition of all transported particles. 4. Fine sediments within macrophyte beds contained high concentrations of organic matter, carbon, nitrogen and phosphorus. The wide scatter in the relationships between mineral grain size and the content of organic matter and nutrients reflects the spatial and temporal complexity of erosion, transport and sedimentation of mineral and organic particles. 5. Enrichment of sediment within macrophyte beds relative to the surrounding substratum ranged from 780 g organic matter m^–2, 30 g N m^–2 and 25 g P m^–2 for the flow-resistant dense canopies af Callitriche cophocarpa to 150 g organic matter m^–2, 6.6 g N m^–2 and 3.4 g P m^–2 for the open canopies of Sparganium emersum. Retention of nutrient-rich particles within the macrophyte beds is probably of limited importance for plant growth in most lowland European streams, because macrophyte growth is rarely nutrient limited.     

Community structure and demography of small-mammal populations in insular montane forests in southern India

bstract Small mammals were studied in insular montane forest patches in the Upper Nilgiris in southern India from February 1994 to September 1996. Nine patches were selected at two sites, one with a single large 600-ha patch, the other with several small patches ranging in size from 0.2 to 60 ha. The population characteristics and community structure of small mammals were studied in relation to patch size and habitat structure within the patches. The two most abundant species were Rattus rattus (2–36 individuals/ha) and Suncus montanus (0–11 individuals/ha), while the abundance of seven other species recorded were very low. The population characteristics of R. rattus examined were density, biomass, proportions of juveniles, sub-adults and adults, mean weight of all animals, mean weight of adults, sex ratio and persistence. The proportion of adults, sub-adults, mean weight of animals, and mean weight of adult males were correlated with patch size. Persistence of sub-adult females in the large patch was especially high. The density and biomass of other small-mammal species were also studied. Two indices of diversity, species richness and proportion of R.␣rattus were compared as measures of community structure. Seven habitat characteristics were measured; of these, canopy cover, canopy height and tree density were correlated with the size of the patch. Density and biomass of species other than R. rattus and proportion of R. rattus were correlated with canopy height. Density and biomass of species other than R. rattus were highest in smaller patches. While the population characteristics of R. rattus may be affected by patch size, the density of rare species may be influenced by factors related to lower canopy height. Migration between patches may be an important factor in maintaining populations in these patches. Received: 5 January 1998 / Accepted: 23 March 1998     

Enhanced particle-feeding capacity of corals on turbid reefs (Great Barrier Reef, Australia)

Reef corals occur across a wide range of habitats, from offshore clear waters to nearshore sediment-laden environments. This study tests the hypothesis that corals from turbid nearshore areas have greater capacity to utilise suspended sediment as a food source than conspecifics from less turbid and midshelf areas. The hypothesis was tested on two common and widespread coral species on the Great Barrier Reef (Pocillopora damicornis and Acropora millepora). The particle clearance rates of samples from more turbid reefs were two-fourfold those of conspecifics from less turbid and midshelf reefs. Rates of sediment ingestion were generally a linear function of sediment load indicating no significant saturation within the concentration range of 1–30 mg dry weight l⁻¹. Estimated assimilation efficiency of particulate ¹⁴C varied between 50 and 80%, and was maximised for midshelf A. millepora at the lowest sediment concentration, suggesting that heterotrophy is more efficient in oligotrophic habitats. Based on feeding-response curves, assimilation efficiencies, and published records of ambient particle concentrations, representatives of these species on turbid inshore reefs are 10–20 times more heterotrophic on suspended sediment than their conspecifics on less turbid and midshelf reefs. Accepted: 7 September 1999     

Colonization of leaf patches at topographically different locations by insect shredders in a small mountain stream

We examined physical constraints on the colonization of leaf patches by shredder individuals by comparing the colonizations of artificially standardized leaf patches placed at different locations within a stream reach (i.e., riffles, middles and edges of pools). Stonefly taxa (Nemoura, Protonemura) colonized riffle patches 2–10 times more often than pool (middle, edge) patches, whereas caddisfly taxa (two species of Lepidostoma, Nothopsyche) almost exclusively colonized pool patches. Colonization also differed between the middle and edge patches in pools for most taxa; it was 2–5 times greater in edge patches for Nemoura and in middle patches for Lepidostoma. The abilities of species to cope with low oxygen circulation and high shear stress appear to determine differences in colonization between riffle and pool patches, whereas species-specific dispersion behavior (e.g., return time from drift) may differentiate colonization between middle and edge patches in pools. Our results suggest that changes in leaf distribution within a reach can affect the suitability of stream reaches in terms of food acquisition for shredder individuals.     

Oxygen and nutrient dynamics of the upside down jellyfish (Cassiopea sp.) and its influence on benthic nutrient exchanges and primary production

The oxygen and nutrient dynamics of the zooxanthellate, upside down jellyfish (Cassiopea sp.), were determined both in situ and during laboratory incubations under controlled light conditions. In the laboratory, Cassiopea exhibited a typical Photosynthesis–Irradiance (P–I) curve with photosynthesis increasing linearly with irradiance, until saturation was reached at an irradiance of ~400 μE m⁻² s⁻¹, with photosynthetic compensation (photosynthesis = respiration) being achieved at an irradiance of ~50 μE m⁻² s⁻¹. Under saturating irradiation, gross photosynthesis attained a rate of almost 3.5 mmol O₂ kg WW⁻¹ h⁻¹, whereas the dark respiration rate averaged 0.6 mmol O₂ kg WW⁻¹ h⁻¹. Based upon a period of saturating irradiance of 9 h, the ratio of daily gross photosynthesis to daily respiration was 2.04. Thus, photosynthetic carbon fixation was not only sufficient to meet the carbon demand of respiration, but also to potentially support a growth rate of ~3% per day. During dark incubations Cassiopea was a relatively minor source of inorganic N and P, with the high proportion of NO _X (nitrate + nitrite) produced indicating that the jellyfish were colonised by nitrifying bacteria. Whereas, under saturating irradiance the jellyfish assimilated ammonium, NO _X and phosphate from the bathing water. However, the quantities of inorganic nitrogen assimilated were small by comparison to carbon fixation rates and the jellyfish would need to exploit other sources of nitrogen, such as ingested zooplankton, in order to maintain balanced growth. During in situ incubations the presence of Cassiopea had major effects on benthic oxygen and nutrient dynamics, with jellyfish occupied patches of sediment having 3.6-fold higher oxygen consumption and 4.5-fold higher ammonium regeneration rates than adjacent patches of bare sediment under dark conditions. In contrast at saturating irradiance, jellyfish enhanced benthic photosynthetic oxygen production almost 100-fold compared to the sediment alone and created a small sink for inorganic nutrients, whereas unoccupied sediment patches were sources of inorganic nutrients to the water column. Overall, Cassiopea greatly enhanced the spatial and temporal heterogeneity of benthic fluxes and processes by creating “hotspots” of high activities which switched between being sources or sinks for oxygen and nutrients over diurnal irradiance cycles, as the metabolism of the jellyfish swapped between heterotrophy and net autotrophy.     

Depth-related changes to density,diversity and structure of benthic megafaunal assemblages in the Fimbul ice shelf region,Weddell Sea,Antarctica

The depth-related patterns in the benthic megafauna of the NE Weddell Sea shelf at the edge of the Fimbul Ice Shelf were investigated at seven sites using towed camera platform photographs. Megafaunal density decreased with depth from 77,939 ha⁻¹ at 245 m to 8,895 ha⁻¹ at 510 m. While diversity was variable, with H′ ranging between 1.34 and 2.28, there were no depth related patterns. Multivariate analyses revealed two distinct assemblages; a shallow assemblage with dense patches of suspension feeders in undisturbed areas and a deep assemblage where these were not present. Disturbance from icebergs explained many observed patterns in faunal distribution. In shallow waters probable effects of disturbance were observed as changes in successional stages; in deeper waters changes in habitat as a result of past disturbance explained faunal distributions. In deeper areas ice ploughing created a mosaic landscape of fine and coarse sediments. Total megafaunal density was highest in areas of coarse sediment (up to 2.9 higher than in finer sediment areas) but diversity was highest in intermediate areas (H′ = 2.35).     

Re-use of shallow sediment patches by Pacific sand lance (<Emphasis Type="Italic">Ammodytes hexapterus</Emphasis>) in Barkley Sound,British Columbia,Canada

We determined whether, during the summers of 2002–2004, young-of-the-year (YOY) Pacific sand lance (Ammodytes hexapterus) re-used patches of sediment in intertidal and shallow subtidal regions of Barkley Sound, British Columbia, Canada. We examined re-use weekly at a single sediment patch in 2003 and 2004 via mark-recapture methods, monthly by re-sampling 15 patches in 2002 and interannually by re-sampling 36 patches in all 3 years (2002–2004). The mark-recapture results showed that, within each summer, batch marked YOY sand lance re-used the same patch up to 8 weeks later. The re-sampling of 15 patches over 4 months in 2002 showed that YOY sand lance consistently occupied patches within a season. We found the greatest variability in sediment patch re-use at the interannual time scale with major fluctuations in occurrence and abundance during 2002–2004. The interannual variability in patch re-use is most likely linked to abundance rather than the local or regional environmental characteristics that we examined.     

Effects of the exotic invader Musculista senhousia on benthic communities of two Mediterranean lagoons

The impact that the alien mussel Musculista senhousia has on benthic biodiversity and community structure was investigated in two Italian transitional environments, where the species was particularly abundant: the Sacca di Goro (Adriatic Sea), and the Rio Padrongiano Deltaic area (Sardinia, Tyrrhenian Sea). Sampling campaigns were conducted by sampling mussel beds from each of two different patch size classes (small: <1.0 m², and large: >10.0 m²), and the surrounding bare sediments. At both study sites, mussel density varied greatly with time within and between patches. Despite the huge difference in benthic species complement at Goro and Padrongiano (33 and 115 taxa, respectively), at both sites the presence of mussel patches had a gross positive effect on the benthic community. At Goro, many trophic guilds showed a positive relationship with mussel abundance. At Padrongiano, a continuous and directional process of patches colonization (both large and small) through time seemed evident, while at Goro the process was interrupted by high-summer mortalities. At both study sites, M. senhousia played a double role: (1) as secondary substratum, enhancing the environmental structural complexity, and (2) as densely aggregated, living organisms, thus giving rise to a number of possible interactions with other invertebrates. The outcome, however, was strictly dependent by the level of anthropogenic stress, in this case water oxygen deficiency, which was different at the two study sites. Guest editors: A. Razinkovas, Z. R. Gasiūnaitė, J. M. Zaldivar & P. Viaroli European Lagoons and their Watersheds: Function and Biodiversity     

互花米草入侵对大型底栖动物群落垂直结构的影响

2012年8月,在浙江省玉环县漩门湾国家湿地公园,利用自制分层采样器,以5 cm为单位对0—25 cm泥层进行分层取样,研究潮间带互花米草滩涂和自然滩涂两种生境中大型底栖动物的垂直分布情况,以及互花米草入侵对大型底栖动物垂直结构的影响。两种生境中共获得大型底栖动物40种,其中软体动物18种,甲壳动物13种,环节动物4种,其他类群5种。在自然滩涂生境中获得的物种数为30种,互花米草生境为31种,两种生境的共有种有21种。互花米草和自然滩涂生境平均栖息密度分别为222.6株/m2和1052.8株/m2。研究结果表明:(1)大型底栖动物主要栖息在0—10 cm泥层深度;(2)大型底栖动物群落沿泥层深度存在分层分布现象;(3)互花米草入侵缩短了底栖动物的垂直分布距离,但对其主要栖息深度影响不大;(4)互花米草入侵改变了原来生境特征,导致大型底栖动物的群落结构发生变化。     

Habitat use,feeding and territorial behavior of a Brazilian endemic damselfish <Emphasis Type="Italic">Stegastes rocasensis</Emphasis> (Actinopterygii: Pomacentridae)

This paper presents the results of the first field study of the Brazilian endemic damselfish Stegastes rocasensis. Underwater data were collected at the Fernando de Noronha Archipelago (NE Brazil), where four sites were visually assayed for local territorial and feeding behaviors, as well as habitat use. S. rocasensis preferentially fed on turf and macroalgae, and during the afternoon period. Its territory size ranged from 0.26 to 13.86 m² (1.61 ± 0.23 m²). Our findings suggest that adults preferably inhabit shallow, turf-rich areas, while sub-adults and juveniles occupy deeper areas with higher percent cover of invertebrates and hard corals, respectively. With regards to agonistic behaviors, pursuit of intruders was the most frequent reaction recorded (p < 0.001); the highest level of aggression was elicited by territorial herbivores (p < 0.001). Nevertheless, analysis of attacks elicited by different fish species within S. rocasensis’ territory showed that more attacks were launched on conspecifics than on other fish species.     

Correlation of the distribution ofMesopodopsis slabberi (Crustacae,Mysidacea) with physico-chemical gradients in a partially-mixed estuary (Tamar,England)

The physical and chemical processes operating in the River Tamar Estuary (south-west England) have been comprehensively described and reported in the literature. There are well-established gradients of salinity, suspended sediment and oxygen which vary both on short-term (tidal) and long-term (seasonal) cycles. Freshwater runoff, the main factor determining salinity distribution, is also the cause of the high variability in suspended sediment concentrations. The biological processes are less well studied and information on the link between the benthic and pelagic systems is particularly lacking. Mysids, through their role as detritivores and as a major component in the diet of some fish, provide this link. Of the four species of mysid distributed longitudinally in the Tamar Estuary, the most abundant isMesopodopsis slabberi which occurs between 5 and 25 km from the estuary head. Observations over an annual cycle have shown marked seasonal changes in both abundance and distribution in the estuary. During winter and spring, densities remained generally low (<50 m⁻³) but, as water temperatures increased, the density increased and reachedca 1200 individuals m⁻³ in July. There was a shift in the longitudinal distribution ofM. slabberi in response to changes in the position of the salinity gradient. Adults comprised the majority of the population in salinities less than 10‰ whereas juveniles and immature animals were distributed over a wider area than the adults and occurred in water of higher salinity than the main adult distribution.M. slabberi appears to utilise the two-layered estuarine circulation to maintain its position in the estuary.     

Host tolerance,not symbiont tolerance,determines the distribution of coral species in relation to their environment at a Central Pacific atoll

Tolerance of environmental variables differs between corals and their dinoflagellate symbionts (Symbiodinium spp.), controlling the holobiont’s (host and symbiont combined) resilience to environmental stress. However, the ecological role that environmental variables play in holobiont distribution remains poorly understood. We compared the drivers of symbiont and coral species distributions at Palmyra Atoll, a location with a range of reef environments from low to high sediment concentrations (1–52 g dry weight m⁻² day⁻¹). We observed uniform holobiont partnerships across the atoll (e.g. Montipora spp. with Symbiodinium type C15 at all sites). Multivariate analysis revealed that field-based estimates of settling sediment predominantly explained the spatial variation of coral species among sites (P < 0.01). However, none of the environmental variables measured (sedimentation, temperature, chlorophyll concentration, salinity) affected symbiont distribution. The discord between environmental variables and symbiont distributions suggests that the symbionts are physiologically tolerant of the variable environmental regime across this location and that the distribution of different host–symbiont combinations present is largely dependent on coral rather than Symbiodinium physiology. The data highlight the importance of host tolerance to environmental stressors, which should be considered simultaneously with symbiont sensitivity when considering the impact of variations in environmental conditions on coral communities.     

高原鼠兔和高原鼢鼠种群暴发区干扰斑块空间分布格局与演替规律

高原鼠兔(Ochotona curzoniae)和高原鼢鼠(Myospalax baileyi)是两种在高寒草甸广泛分布的小型哺乳动物,其暴发活动可能造成高寒草甸的严重退化。使用无人机分别拍摄典型的高原鼠兔和高原鼢鼠种群暴发区图像,解译高原鼠兔洞口、高原鼢鼠土丘和它们的干扰斑块,获得干扰斑块的周长和面积信息,计算干扰斑块形状指标,确定高原鼠兔与高原鼢鼠暴发活动对高寒草甸的影响。研究表明：(1)高原鼠兔与高原鼢鼠暴发活动形成的干扰斑块周长与面积比(PARA)、形状指数(SHAPE)和分形维数(FRAC)差异显著(P<0.05),其中高原鼢鼠斑块的FRAC是1.0124±0.0091,属于普通的规则几何圆形分布格局,高原鼠兔斑块的FRAC是1.1203±0.0546,属于分形几何不规则的分布格局。(2)高原鼠兔洞口数量与其斑块面积没有线性相关关系(P=0.9677),高原鼢鼠土丘数量与其斑块面积呈显著正相关(P<0.05)。(3)高原鼢鼠斑块面积统计学上呈长尾分布,不符合正态分布(P<0.01)。根据高原鼠兔斑块面积分布格局可将斑块演替分为三个阶段：土丘形成阶段、斑块连通...     

Short-term sedimentation in Tagus estuary,Portugal: the influence of salt marsh plants

Short-term sediment deposition was studied at four salt marsh areas in the Tagus estuary. In areas covered with Sarcocornia perennis, Sarcocornia fruticosa, Halimione portulacoides and Spartina maritima and also in the non-vegetated areas, sedimentation was measured as the monthly accumulation of sediments on nylon filters anchored on the soil surface, from August 2000 to May 2001. Our experiments were used also to determine the influence of the different plant species in vertical accretion rates. Short-term sedimentation rates (from 2.8 to 272.3 g m⁻² d⁻¹) did show significant differences when the four salt marshes studied in the Tagus estuary were compared to each others. Salt marshes closer to the sediment sources had higher sedimentation rates. Our results suggest that the salt marsh type and surface cover may provide small-scale variations in sedimentation and also that sediment deposition values do change according to the position of the different plant species within the salt marsh. Sedimentation is an essential factor in salt marsh vertical accretion studies and our investigation may provide support to help forecast the adaptative response of the Tagus estuary wetlands to future sea level rise.     

Urea Transformation of Wetland Microbial Communities

Transformation of urea to ammonium is an important link in the nitrogen cycle in soil and water. Although microbial nitrogen transformations, such as nitrification and denitrification, are well studied in freshwater sediment and epiphytic biofilm in shallow waters, information about urea transformation in these environments is scarce. In this study, urea transformation of sedimentary, planktonic, and epiphytic microbial communities was quantified and urea transformation of epiphytic biofilms associated with three different common wetland macrophyte species is compared. The microbial communities were collected from a constructed wetland in October 2002 and urea transformation was quantified in the laboratory at in situ temperature (12°C) with the use of the ¹⁴C-urea tracer method, which measures the release of ¹⁴CO₂ as a direct result of urease activity. It was found that the urea transformation was 100 times higher in sediment (12–22 mmol urea-N m⁻² day⁻¹) compared with the epiphytic activity on the surfaces of the submerged plant Elodea canadensis (0.1–0.2 mmol urea-N m⁻² day⁻¹). The epiphytic activity of leaves of Typha latifolia was lower (0.001–0.03 mmol urea-N m⁻² day⁻¹), while urea transformation was negligible in the water column and on the submerged leaves of the emergent plant Phragmites australis. However, because this wetland was dominated by dense beds of the submerged macrophyte E. canadensis, this plant provided a large surface area for epiphytic microbial activity—in the range of 23–33 m² of plant surfaces per square meter of wetland. Thus, in the wetland system scale at the existing plant distribution and density, the submerged plant community had the potential to transform 2–7 mmol urea-N m⁻² day⁻¹ and was in the same magnitude as the urea transformation in the sediment.