Diversity patterns of free‐living marine nematodes in the southwest continental shelf off Bay of Bengal and their link to abiotic variables

Marine sediments in continental shelf ecosystems harbor a rich biodiversity of benthic communities. In this study, the spatial and temporal diversity and community assemblages of free‐living marine nematodes were studied by sampling at six depths and over 3 years from the southwest continental shelf off Bay of Bengal, one of the least explored tropical shelf ecosystems. The dominant marine nematode species were related with abiotic variables as part of this study. The effects of sediment granulometry generally decreased with increasing depth and the highest nematode density and species diversity were recorded on coarse sand (shallower depths). Multivariate analysis of the nematode community data showed that community structure differed significantly among depths as well as among years. Statistical analyses showed significant correlations between the nematode community and abiotic variables. Sediment texture, organic matter, water pressure and depth profile were crucial factors for determining diversity, vertical profile and feeding types of the nematode community. Other environmental factors, including anthropogenic pressure, did not have an effect on nematode diversity except for the presence of some tolerant species (Metachromadora spp., Sabatieria spp. and Siplophorella sp.). This study represents a baseline of knowledge of free‐living marine nematode communities that can be used in the future to compare nematode assemblages from temperate shelf ecosystems.     

Sand-mediated divergence between shallow reef communities on horizontal and vertical substrata in the western Indian Ocean

Distinctions are rarely made between vertical and horizontal surfaces when assessing reef community composition, yet physical differences are expected because of hydrodynamic differences and sediment accumulation on flat surfaces. As sand often diminishes biotic cover, we hypothesised that vertical surfaces will support a greater biomass but have lower diversity due to domination by a few species. To test this, we quantified sessile communities on vertical and horizontal surfaces at three sites in the Delagoa Bioregion on the east coast of South Africa. Community composition consistently differed: vertical communities were dominated by various filter feeders, especially the ascidian Pyura stolonifera, whereas those on horizontal reef comprised a mixture of filter feeders and various algae. The total number of species and all diversity metrics were significantly greater for horizontal reef surfaces. Contrastingly, Simpson’s dominance and biomass were significantly greater for vertical reef surfaces. Percentage cover of sand explained much of the variation in community composition whereas depth did not. Small-scale topographic differences in substratum orientation associated with differences in sand inundation will therefore influence both α and β diversity. Coastal developments and activities that alter sand movements and delivery to the coastal zone are therefore likely to have a profound influence on the maintenance and diversity of shallow subtidal communities.     

Taxon‐related diversity patterns from the continental shelf to the slope: a case study on nematodes from the Western Indian Ocean

A study was carried out across the continental shelf and slope in the Western Indian Ocean along the Kenyan margin to unravel depth‐related species diversity patterns portrayed by different nematode families. Sediment samples were collected along four bathymetric transects at 20, 50, 200, 500, 1000 and 2000 m. Three nematode families were selected for species analysis, based on their general occurrence with relatively high numbers and diversity from shelf to slope. All three families exhibited a distinctly different shelf and slope nematode species community. However, all three families also had a significant proportion of species that displayed a eurybathic distribution. Coincidentally, Microlaimidae, the least species‐rich family had the most eurybathic species (75%) compared to Comesomatidae (39%) and Chromadoridae (32%). Total number of species per depth zone along the sampled area (gamma diversity) was three to four times the average number of species (alpha) per station. The difference was more pronounced at the slope than at the shelf. The species turnover was also higher at the slope than at the shelf stations. Each of the three families had a different diversity pattern: Comesomatidae showed a peak at mid‐depth, Chromadoridae increased with depth, and Microlaimidae showed no prominent change with depth. When the three families were combined, the shelf maintained a lower diversity compared to the slope (both local and regional). There was no consistency between genus and species diversity patterns with depth, indicating that genus diversity data may not be a proxy for species diversity. At the lowest taxonomic level (species), the slope showed a higher diversity than the shelf, whereas at higher (genus) taxonomic level the diversity was comparable between the slope and the shelf. The number of species encountered increased with the number of samples analysed and did not reach asymptote, meaning that the area was still under‐sampled. This situation points to the possibility of an even higher regional diversity.     

Meiofauna distributions at the oxygen minimum zone in Changjiang (Yangtze) River Estuary waters

1 IntroductionOxygen minimum zone (OMZ) is a midwaterregion in the open ocean where dissolved oxygen con-centrations fall below 0.5 ml/L (0.714 mg/dm3,Kamykowski and Zentara, 1990; Levin et al.,1991). It results from the biological oxygen demandand is wel…     

Meiofauna in a cold-seep community off Hatsushima,central Japan

The community structure of the bathyal meiofauna of a cold-seep community found off Hatsushima in Sagami Bay, central Japan, was compared with the community composition outside the influence of the seep, using sediments collected during dives 226 and 227 of the deep-sea submersibleShinkai 2000. The sediment from the Hatsushima seep site (HSS) was very coarse, black in color, and with an odor of hydrogen sulfide, suggesting reduced thiobiotic conditions. The sediment from the control area was well-oxygenated, fine silt. Despite the differences in the characteristics of the sediments, the abundance of meiofauna in the HSS was not very different from that in the control area. However, its composition even at the major taxonomic group level was distinct; for example, a high nematode/copepod ratio occurred in one of the samples collected at the HSS. At the species level, nematodes were less diverse at the HSS than at the control area. The composition of the nematode fauna at the HSS showed stronger affinity with that collected at the adjacent control area than with a community sampled from other deep-sea environments or another seep community in shallow water. This emphasizes that the adaptation of nematodes to the thiobiotic condition is controlled by local conditions.     

Density,biomass and community structure of demersal fishes off the Pacific coast of northeastern Japan

Demersal fish communities were studied on the lower continental shelf and the upper continental slope along the Pacific coast of northeastern Japan. Species composition, number and weight of each species were examined based on otter trawl samples at 45 stations. Mean density and biomass of demersal fishes were 131 ha^–1 and 21 kg ha^–1, respectively. The ten most abundant species comprised of about 95% of total number and weight of overall catch indicating simple species composition. Gadiform fishesTheragra chalcogramma, Gadus macrocephalus andPhysiculus maximowiczi were the most important species by number, weight and frequency of occurrences, and three main community types represented by the three key species were recognized.Theragra-dominant community showed higher density and biomass, and lower diversity thanPhysiculus-dominant community did. Species diversity of demersal fish community was negatively correlated to density and biomass. Density and biomass of demersal fish community were high on the uppermost slope, and the high abundance resulted from low-diversity communities dominated byT. chalcogramma andG. macrocephalus.     

Temporal and spatial variation in the Nazaré Canyon (Western Iberian margin): Inter-annual and canyon heterogeneity effects on meiofauna biomass and diversity

The Nazaré Canyon on the Portuguese Margin (NE Atlantic) was sampled during spring-summer for three consecutive years (2005–2007), permitting the first inter-annual study of the meiofaunal communities at the Iberian Margin at two abyssal depths (~3500 m and ~4400 m). Using new and already published data, the meiofauna standing stocks (abundance and biomass) and nematode structural and functional diversity were investigated in relation to the sediment biogeochemistry (e.g. organic carbon, nitrogen, chlorophyll a, phaeopigments) and grain size. A conspicuous increase in sand content from 2005 to 2006 and decrease of phytodetritus at both sites, suggested the occurrence of one or more physical disturbance events. Nematode standing stocks and trophic diversity decreased after these events, seemingly followed by a recovery/recolonisation period in 2007, which was strongly correlated with an increase in the quantity and bioavailability of phytodetrital organic matter supplied. Changes in meiofauna assemblages, however, also differed between stations, likely because of the contrasting hydrodynamic and food supply conditions. Higher meiofauna and nematode abundances, biomass and trophic complexity were found at the shallowest canyon station, where the quantity, quality and bioavailability of food material were higher than at the deeper site. The present results suggest that even though inter-annual variations in the sedimentary environment can regulate the meiofauna in the abyssal Nazaré Canyon, heterogeneity between sampling locations in the canyon were more pronounced.     

Meiofauna of the western continental shelf of India,Arabian Sea

Meiofaunal standing stock and nematode community structure were investigated in the western continental shelf of India by collecting samples from every degree square of the shelf during two cruises of the FORV (Fishery and Oceanographic Research Vessel) Sagar Sampada, conducted in 1998 and 2001. Samples were collected from 30, 50, 100 and 200 m depths using a Smith Mc Intyre grab. Meiofaunal density ranged from 8 Ind. 10 cm⁻² to 1208 Ind. 10 cm⁻² and biomass from 0.07 mg 10 cm⁻² to 6.11 mg 10 cm⁻². Nematodes were the dominant meiofaunal group, contributing 88% of the density and 44% of the biomass. Harpacticoid copepods were the second important taxa, contributing 8% of both biomass and density. Altogether, 154 species of nematodes belonging to 28 families were recorded from the study area. Numerically, Desmodora spp., Dorylaimopsis sp., Tricoma spp., Theristus spp. and Halalaimus spp. were the dominant species. In general, there was a decrease in biomass and density of meiofauna and species diversity of nematodes with increase in depth. There was a 67% drop in species number from 51 to 100 m (106 species) to the shelf edge (35 species). Species richness and diversity indices showed consistent decrease with depth. The species dominance index was higher below 150 m depth. ANOSIM (from PRIMER) showed a significant difference between the nematodes of the near shore and shelf edge. Latitudinal variation was observed only in the number of nematode species. Biomass and abundance of nematodes were found to increase from coarse to fine sediment, while copepods showed an opposite trend. Multivariate analyses of nematode communities did not reveal any latitudinal or substratum differences. Variables such as depth, latitude, organic matter (OM) and amount of clay were the most relevant parameters influencing the biomass and density of meiofauna, while depth and temperature were the important parameters explaining the distribution of the nematode communities along the western Indian shelf.     

Deep-sea nematode assemblage has not recovered 26 years after experimental mining of polymetallic nodules (Clarion-Clipperton Fracture Zone, Tropical Eastern Pacific)

We investigated nematode assemblages inhabiting the 26-year-old track created by experimental deep-sea mining of polymetallic nodules, and two adjacent, undisturbed sites, one with nodules and one without nodules. The aim was to compare density, assemblage structure, and diversity indices in order to assess the process of recovery of the nematode assemblage inhabiting the disturbed site. This experimental dredging was conducted in 1978 by the Ocean Minerals Company (USA) in the area of a French mining claim in the Clarion-Clipperton Fracture Zone (Tropical Eastern Pacific) at a depth of about 5000 m. The nematode assemblage had not returned its initial state 26 years after the experimental dredging: the total nematode density and biomass within the dredging track were significantly lower than outside the track; the biodiversity indices showed significantly lower nematode diversity within the track; and the structure of the nematode assemblage within the track differed significantly from those in the two undisturbed sites outside the track. However, there were no significant differences in the mean body volumes of adult nematodes and adult-juvenile ratios between the track and reference sites. Parameters such as the rate of sediment restoration (which depends on local hydrological conditions) and the degree and character of the disturbance appeared to be of considerable importance for the recovery rate of the deep-sea nematode assemblages and their ability to recolonize disturbed areas. The rates of recolonization and recovery may vary widely in different deep-sea regions.     

渤海自由生活海洋线虫多样性的研究

首先介绍Warwick和Clarke(1995)提出的等级生物多样性指数以及在自由生活海洋线虫中的应用,重点以1998年9～10月和1999年4～5月两个航次的沉积物样品,用物种数目(S)、Margalef物种丰富度指数(d)、优势度Simpson指数(λ)、多样性Shannon-Wiener指数(H')、Pielou的均匀度指数(J)、Hill多样性系列指数(N₁,N₂和N_+∞)、分类多样性指数(△)、分类差异指数(△*)、等级多样性指数(△+)对渤海自由生活海洋线虫的多样性进行了研究。结果表明,黄河输入的泥沙对渤海自由生活海洋线虫多样性的变化有一定的作用,距离黄河口越近的站位,群落具有的物种多样性和均匀性越低,优势度越高;距离黄河口越远的站位,群落具有的物种多样性和均匀性越高,优势度越低。     

Benthic habitat quality change as measured by macroinfauna community in a tidal flat on the west coast of Korea

Yellow Sea tidal flats are internationally recognised for their contribution to biological diversity and yet are under enormous pressure from reclamation, pollution and overexploitation. The benthic macroinfauna community is the dominant community on these tidal flats and a reliable indicator of benthic environmental changes. We surveyed the current benthic macroinfauna community of the Ganghwa Southern Tidal Flat, the largest remaining Korean mud flat in the Yellow Sea, in order to examine changes in the environmental situation of this benthic ecosystem. The results show a significant decline in species diversity from the last survey made in 2003, and a shift in species composition with appearances of polychaetes indicative of pollution and physical disturbances and other opportunistic species becoming dominant in both density and biomass. The benthic community shift observed during the two study periods may be associated with increased nutrient pollution as well as increased physical disturbances in this area. However, we recognise the limitations of the data both in frequency and scope but believe the significant changes to the composition of the benthic fauna are sufficient to warrant concern. Observations are required to examine the extent to which these human activities induce benthic community shift in this tidal flat.     

Seasonal variations in the community structures of macrobenthic fauna and their health status in an estuarine bay,Gwangyang Bay in Korea

Macrobenthic fauna in an estuarine Gwangyang Bay, southern Korean coast, were investigated to uncover recent variations in their community structures. In the study area, macrobenthic faunal communities were mainly composed of polychaete worms which were the most abundant faunal group with the highest values in species number and density, while mollusks accounted for the highest proportion in total biomass. There was no clear seasonal difference in species richness during the two year period of the investigation, but the mean density and biomass increased every spring and summer due to the mass recruitment of Theora fragilis. The Shannon’s diversity index (H') was more than 2.0 during most sampling seasons and did not show any significant seasonal difference except for the data in August, 2011 when azoic conditions occurred. The community structures of macrobenthos in Gwangyang Bay did not show any remarkable change in the dominance of the two top dominant species, Scoletoma longifolia and Heteromastus filiformis, which abundantly occurred in all seasons, except for the abundance peaks associated with high occurrence of T. fragilis and Paraprionospio cordifolia, especially in spring and summer and in autumn, respectively. These fauna changes reflected the changes in the macrobenthic community health status in Gwangyang Bay, where stable conditions and a healthy status prevailed in winter, but a slightly disturbed status prevailed from spring to autumn.     

渤海大型底栖动物种类组成与群落结构研究

本文利用2008年8月在渤海23个站位采集的样品对大型底栖动物的丰度、生物量、群落结构及其与环境因子的关系进行了研究。共鉴定出大型底栖动物300种,主要类群包括环节动物多毛类、节肢动物甲壳类、软体动物、棘皮动物等。研究海域大型底栖动物的平均丰度为1 094.7ind/m2,平均生物量为11.78g/m2,其中丰度最高区位于辽东湾,最低区位于渤海湾;生物量最高区位于渤海中部,最低区出现在渤海海峡湾口处。依据丰度和生物量进行了CLUSTER聚类和MDS标序图分析,结果可将研究站位划分为6个站组,均与沉积物底质类型密切相关。所研究海域底质类型包括黏土质粉砂、砂、砂质粉砂、粉砂质砂、砂-粉砂-黏土5种。通过BIOENV分析,发现影响大型底栖动物群落结构和分布的主要环境因子有水深、黏土粉砂含量、脱镁叶绿酸含量、叶绿素含量等。与20世纪90年代历史数据的对比发现,本次研究大型底栖动物的物种数目明显减少,平均丰度也减少,特别是优势物种表现出了小型化趋势,传统的大型种类,如心形海胆、双壳类等,被小个体的多毛类和甲壳类所取代。本研究表明渤海近年来受人类活动影响,出现了不同程度的污染,导致大型底栖动物群落结构发生了变化。     

浙江瓯江口海域游泳动物群落特征及其与环境因子的关系

瓯江口海域是传统的浙江省渔业资源栖息地,现今受高强度人为扰动的影响。为了解瓯江口海域游泳动物群落组成和多样性特征,采用单拖网方法对瓯江口春季(3月)和秋季(11月)游泳动物进行调查,并对其群落组成、生物多样性、资源密度和时空分布格局及主要影响因子等进行了研究。调查结果显示,研究海域共采集到游泳动物78种,其中鱼类有36种、甲壳动物28种、头足类3种。游泳动物优势种春季有9种,秋季有6种;两个季节共同优势种有4种。春季的渔获物的丰度远低于秋季,而春季的生物量则与秋季相差不大。基于丰度和生物量的多样性指数(H’)、均匀度指数(J’)和丰富度指数(d)均为秋季高于春季。在瓯江口海域,水体悬浮物是影响游泳动物群落多样性和丰富度的最重要的环境因子。通过对浙江瓯江口海域游泳动物群落特征及其与环境因子的内在关联的研究,有望为瓯江口海域渔业资源变迁特征和资源养护管理决策提供基础数据和科学依据。     

Macrofaunal nematodes of the deep Whittard Canyon (NE Atlantic): assemblage characteristics and comparison with polychaetes

Large nematodes form an important component of deep‐sea macrofaunal assemblages, but are often considered to be part of the meiobenthos and are rarely studied. We analysed the standing stocks, diversity, and functional group and genus‐level composition of macrofaunal nematodes at lower bathyal depths (3,500 m water depth) in the Whittard Canyon system (NE Atlantic) and on the adjacent continental slope. Five replicate sample sets were obtained using a Megacorer, at four locations (three canyon branches, one slope). Sediment samples were processed on a 500‐μm mesh to provide both nematode and polychaete data from the same samples. The dominant nematode genera included Paramesacanthion, Metacylicolaimus, Cylicolaimus and Phanodermopsis. Nematode standing stocks (density and biomass) increased significantly from slope to canyon locations. Similarly, nematode dominance increased substantially (and diversity decreased) from slope to canyon locations. Nematode feeding groups and tail shape groups also appeared to exhibit common trends across study locations. Nematode genus‐level faunal composition varied significantly between slope and canyon locations. We describe and discuss the broadly similar trends detected in the matching polychaete data, noting some differences in the polychaete density and diversity responses. We suggest that the similar trends in macrofaunal nematode and polychaete ecology across our four study locations reflect responses to both changing sedimentology and food availability.     

象山港人工鱼礁投放对大型底栖动物群落结构的影响

为探明人工鱼礁生境中底栖生物群落结构的组成情况,评估人工鱼礁投放的生态修复效果,根据2006—2007年4季象山港全港区以及2010年4季人工鱼礁区和邻近区的调查资料,对比分析了3个不同区域大型底栖动物群落的差异。共鉴定出大型底栖动物116种,其中多毛类46种,软体动物35种,甲壳类18种,棘皮动物6种,其他类11种,各区域大型底栖动物的主要类群均为多毛类和软体动物。大型底栖动物密度、Shannon-Wiener多样性指数和Margalef丰富度指数表现为邻近区鱼礁区全港区,生物量为鱼礁区邻近区全港区。双因子方差分析结果表明:区域间,大型底栖动物种类数、密度、Shannon-Wiener多样性指数、Pielou均匀度指数和Margalef丰富度指数均存在显著性差异,生物量无显著性差异;季节间,大型底栖动物种类数和密度存在显著性差异,其余参数均无显著性差异。非度量多维尺度排序分析结果显示,鱼礁区和邻近区的大型底栖动物群落相似度较高,而与全港区的相似度较低。相似性百分比进一步揭示,鱼礁区和邻近区大型底栖动物群落间的平均非相似性为77.19%,不倒翁虫(Sternaspis scutata)为造成群落间差异的重要贡献物种;鱼礁区和全港区群落间的平均非相似性高达91.34%,不倒翁虫、薄云母蛤(Yoldia similis)、纵肋织纹螺(Nassarius variciferus)和洼颚倍棘蛇尾(Amphioplus depressus)为重要贡献物种;邻近区和全港区大型底栖动物群落间的平均非相似性也高达91.47%,不倒翁虫、薄片镜蛤(Dosinia laminata)和薄云母蛤为重要贡献物种。双因素相似性分析结果表明,人工鱼礁区、邻近区和全港区大型底栖动物群落季节间存在显著性差异,但区域间无显著性差异。不同区域两两间检验结果表明,人工鱼礁区和全港区的大型底栖动物群落存在显著性差异,人工鱼礁区和邻近区以及邻近区和全港区均无显著性差异。研究结果表明,象山港白石山群岛海域人工鱼礁的投放对大型底栖动物群落结构影响显著,但影响范围局限于人工鱼礁区附近。     

Long‐term nutrient enrichment alters nematode trophic structure and body size in a Spartina alterniflora salt marsh

The effects of long‐term experimental nutrient enrichment on nematode trophic guilds and morphometrics were examined in intertidal saltmarsh creeks of Plum Island Estuary, Massachusetts, USA. Nematodes from the marsh‐edge Spartina alterniflora habitat in a reference creek (n = 3300) were sampled annually and compared with nematodes (n = 3100) from a creek in which nitrate and phosphate loading rates were increased approximately 10–15× for 6 years. Trophic guilds in both creeks were dominated by epistrate (diatom) feeders and predators, and natural temporal variability across years was high. However, after 4 years of nutrient enrichment, a shift in the nutrient‐enriched creek was detected from a dominance of epistrate feeders to an increased proportion of predators, even though neither the benthic microalgae biomass nor the total density of nematodes was affected by fertilization. Nematodes also became longer, and longer relative to their diameter over time with nutrient enrichment because of the shift in trophic structure as short‐stout epistrate feeders were replaced with longer, more slender predators. These changes may have been directly related to nutrient enrichment effects on benthic algae or indirectly to the many other effects of enrichments on ecosystem structure or function. Our research indicates that nutrient enrichment alters the nematode community and this alternation may directly or indirectly affect the response of benthic algae to nutrient enrichment and as well as other ecosystem services.     

Nematode species distribution patterns at the Håkon Mosby Mud Volcano (Norwegian Sea)

The Håkon Mosby Mud Volcano (HMMV) is a methane seep that is densely populated by the bacteria Beggiatoa spp. as well as by tubeworms of the Family Siboglinidae. Analyses of samples from seven stations located in five different habitats (bare sediments, bacterial mats, siboglinid fields, sediments with small patches of siboglinids and areas of ‘normal’ sea floor) revealed differences in the density and species‐level diversity of nematode communities. Total densities increased from stations inside the mud volcano and on its rim towards the ‘normal’ areas outside the volcano. Nematode species diversity was similar in samples from the siboglinid fields and the bare HMMV sediments. However, the sediment with patches of siboglinids collected outside the volcano was characterised by a diverse nematode community comprising 47 species, whereas the control sediments without siboglinids yielded only 27 species. The nematode assemblage at the bacterial mat site included only two identified species, Halomonhystera disjuncta and Sabatieria ornata, with H. disjuncta being strongly dominant. Terschellingia distlamphida, S. ornata and Aponema ninae dominated nematode communities at the siboglinid fields and in bare HMMV sediments. Species dominating at stations inside the caldera were found in comparable low densities in the sediment with siboglinid patches collected outside the volcano, but were missing in the background samples, where species of Endeolophos, Acantholaimus and Desmoscolex were dominant. Species diversity generally decreased with sediment depth. A subsurface‐dwelling nematode community was observed in the siboglinid fields and the bare sediments. Background sediments showed a sharp decline with 18–20 species at 0–2 cm versus three species at 3–5 cm sediment depth. The trophic composition of the nematode fauna at the HMMV showed a prevalence of deposit feeders at almost all stations, whereas background sediments exhibited a uniform distribution of non‐selective and selective deposit feeders and epistrate feeders. The high percentage of deposit feeders inside the mud volcano could be related to the grain‐size distribution and the significantly higher bacterial biomass compared to the control sediments.     

Patterns in subtidal seaweed communities on coral-dominated reefs at Sodwana Bay on the KwaZulu-Natal coast,South Africa

Subtidal seaweed communities of the northern coast of KwaZulu-Natal (KZN) have not been studied before. At Sodwana Bay, we tested the hypotheses that the seaweed communities would (1) differ floristically with depth, (2) be more species-rich in shallower water, (3) show similar biomasses within the depth range sampled and (4) be affected (biomass and/or species composition) by sand. Samples were collected (using SCUBA) from reefs ('sites') at depths of about 1m, 7m, 10m, 15m and 26m. Each sample comprised all macroscopic (non-crustose) seaweeds within a 25cm × 25cm quadrat (five quadrats per depth). Environmental factors, including percentage sand cover and depth, were recorded. The seaweed communities were mainly compact turfs; a total of 82 Rhodophyta, 14 Chlorophyta and eight Phaeophyta were recorded. Ordination (canonical correspondence analysis) and classification (Twinspan) of the data showed clear differences in the floristic composition (either as species biomass or presence/absence) with depth, mainly between shallow subtidal communities (0.5–1.0m depth) and those at intermediate depths (5.5–15.0m), followed by the deepest communities (25.7–29.0m) and those at intermediate depths. The shallow samples showed the greatest species diversity. Total seaweed biomass decreased significantly with depth and percentage bare (seaweed-free) substratum increased significantly with depth, possibly because of lower wave action, light penetration, or different grazing patterns. Sand (as percentage sand cover) also affected seaweed community composition, but this showed no pattern with depth. The algal communities on these reefs at Sodwana Bay showed exceptional α-diversity, with 104 taxa (>20% of the recorded KZN flora) occurring in the total sample area of only 1.56m². The seaweed communities of northern KZN are structurally and floristically similar to those of other tropical coral reefs around the world.     

Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea

The long-term effects of sand extraction on macrozoobenthic communities were investigated in an offshore area in the Northern Adriatic Sea characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. Univariate and multivariate analyses highlighted that the macrozoobenthic community responses to the dredging operations were (1) a rapid initial recolonisation phase by the dominant taxa present before dredging, which took place 6-12 months after sand extraction; (2) a slower recovery phase, that ended 30 months after the operations, when the composition and structure of the communities were similar in the dredged and reference areas. This pattern of recolonisation-recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction.