Physico-chemical variables influencing the distribution and abundance of macroinvertebrates in salt marsh habitats of the Berg River Estuary, South Africa

The Caspian Tern Hydroprogne caspia is widely distributed, but it is rare and little studied in southern Africa. We recorded the prey delivered by Caspian Terns to a breeding colony near the Berg River Estuary on the west coast of South Africa over two seasons (2013 and 2014) using a non-invasive photo-sampling approach. A total of 1 541 prey were identified (90% of prey photographed). At least 18 fish species were recorded, including both marine and estuarine species, indicating that breeding terns foraged in the Berg River Estuary, 7 km from the colony, as well as in St Helena Bay. Southern Mullet Liza richardsonii were the main prey in both seasons (57.6% overall), followed by gobies (15.6%, including Pelagic Goby Sufflogobius bibarbatus), Elf Pomatomus saltatrix (4.7%) and Cape Silverside Atherina breviceps (3.2%). The standard length of mullet ranged from 30 mm to 215 mm (mean ± SD: 101 ± 36 mm). Parents delivered significantly smaller mullet to small chicks than large chicks and partners. The terns lost 3.1% of prey to kleptoparasitism by Kelp Gulls Larus dominicanus (2.6%) and conspecifics (0.5%). Attacks occurred while in flight or as they landed to deliver their prey. Some adults ‘pseudo-swallowed’ prey items while pursued in flight to deter kleptoparasitic attacks. Our study confirms that Caspian Terns exploit a wide range of prey and it is unlikely that the abundance of suitable prey limits their population in the region.

Southern New England salt marsh vegetation and habitats are changing rapidly in response to sea-level rise. At the same time, fiddler crab (Uca spp.) distributions have expanded and purple marsh crab (Sesarma reticulatum) grazing on creekbank vegetation has increased. Sea-level rise and reduced predation pressure drive these changing crab populations but most studies focus on one species; there is a need for community-level assessments of impacts from multiple crab species. There is also a need to identify additional factors that can affect crab populations. We sampled crabs and environmental parameters in four Rhode Island salt marshes in 2014 and compiled existing data to quantify trends in crab abundance and multiple factors that potentially affect crabs. Crab communities were dominated by fiddler and green crabs (Carcinus maenas); S. reticulatum was much less abundant. Burrow sizes suggest that Uca is responsible for most burrows. On the marsh platform, burrows and Carcinus abundance were negatively correlated with elevation, soil moisture, and soil percent organic matter and positively correlated with soil bulk density. Uca abundance was negatively correlated with Spartina patens cover and height and positively correlated with Spartina alterniflora cover and soil shear strength. Creekbank burrow density increased dramatically between 1998 and 2016. During the same time, fishing effort and the abundance of birds that prey on crabs decreased, and water levels increased. Unlike in other southern New England marshes where recreational overfishing is hypothesized to drive increasing marsh crab abundance, we propose that changes in crab abundance were likely unrelated to recreational finfish over-harvest; instead, they better track sea-level rise and changing abundances of alternate predators, such as birds. We predict that marsh crab abundance will continue to expand with ongoing sea-level rise, at least until inundation thresholds for crab survival are exceeded.

Antagonistic effects of a native apple snail on other snails and macroinvertebrates in Southern Pampas waterbodies: A mesocosm approach

Aquatic Ecological Index based on freshwater (ICE(RN-MAE)) for the Rio Negro watershed, Colombia. Available indices to assess the ecological status of rivers in Colombia are mostly based on subjective hypotheses about macroinvertebrate tolerance to pollution, which have important limitations. Here we present the application of a method to establish an index of ecological quality for lotic systems in Colombia. The index, based on macroinvertebrate abundance and physicochemical variables, was developed as an alternative to the BMWP-Col index. The method consists on determining an environmental gradient from correlations between physicochemical variables and abundance. The scores obtained in each sampling point are used in a standardized correlation for a model of weighted averages (WA). In the WA model abundances are also weighted to estimate the optimum and tolerance values of each taxon; using this information we estimated the index of ecological quality based also on macroinvertebrate (ICE(RN-MAE)) abundance in each sampling site. Subsequently, we classified all sites using the index and concentrations of total phosphorus (TP) in a cluster analysis. Using TP and ICE(RN-MAE), mean, maximum, minimum and standard deviation, we defined threshold values corresponding to three categories of ecological status: good, fair and critical.

The study sought to assess the relationship between macroinvertebrate abundance attracted to decomposing leaf litter from three tree species and the physicochemical properties of water in Kapingazi in Embu County, Kenya. Five sites with evident human activities were purposively selected. Fresh leaves were plucked from three plant species (Lantana Camara, Guajava sp., and Eucalyptus grandis), air-dried, and about 5.0 g packed in 12 mesh/litter bags (litter bags) for each species. The physico-chemical variables were measured (mostly in-situ) at the sites. Both correlation and multilinear regression analyses were done to establish the relationship between the macroinvertebrates and the physicochemical properties of the river water. The findings revealed that the macroinvertebrates were more abundant in the dry month (2233) than the wet month (1158) with peak macroinvertebrate abundance recorded on the 14^th and 21^st observation days for all the sites in the months of September and December 2018, respectively. Correlation was positive between macroinvertebrate abundance and pH, dissolved oxygen and temperature. Salinity correlated highly and positively with EC and TDS in both months. Regression revealed increasing pH, DO, Q and TP generally increased macroinvertebrate abundance. It was concluded that macroinvertebrate abundance was affected by site, season, plant species and the physicochemical properties of the water in Kapingazi River. It is recommended that human activities at different sites be regulated with more proactive policy and legal initiatives to improve and stabilize the balance of biotic interactions for a sustainable riverine ecosystem like that of Kapingazi River.

<p>The red mangrove crab, <em>Goniopsis cruentata,</em> influences the recruitment and composition of plant species in the mangrove ecosystem and it is an important fishery resource. Nevertheless, no current management and conservation plans are available for this species for the Brazilian coast. This investigation evaluated the population structure and reproductive biology in populations of <em>G. cruentata</em> under contrasting fishery pressures. The sampling program was carried out in two mangroves, Vaza-Barris and Sergipe River, from January through December 2011. Crabs from both mangroves were randomly collected by a professional fisherman during daytime low tide periods, using a fishing rod baited with pieces of a locally abundant gastropod, <em>Pugilina morio</em>, during 20min/area (catch per unit effort). Monthly measurements of air, sediment surface layer and water temperatures were obtained with a digital thermometer and salinity with an optical refractometer. Both crab populations were compared concerning their abundance, body size, sex ratio, size at onset of sexual maturity and fecundity (FI). Abiotic factors (air, water and mud temperature; and salinity) showed no significant differences between sampling localities. A total of 4 370 crabs were sampled, 2 829 from the Sergipe River and 1 541 from the Vaza-Barris River. The abundance and body size of crabs were compared between mangroves, and statistically significant differences were found. The sex ratio for both populations differed from the expected 1:1 ratio, and a significant deviation in favor of juvenile males was obtained, while adults showed a bias toward females. The estimated size at onset of sexual maturity for both sexes was similar in both populations. However, the populations differed significantly in the number and volume of eggs: a higher FI was obtained in females from the Sergipe River, while a higher egg volume was observed in females from the Vaza-Barris River mangrove. These results indicated a tendency to decrease the body size, the abundance of crabs and the reproductive potential of the species, with higher fishing intensities, reinforcing the need to develop a management plan as well as to establish conservation units for <em>G. cruentata</em> in the Northeastern Brazilian coast. Habitat loss in conjunction with long-term overfishing can have irreversible consequences, which can impact not only the populations of commercially exploited crabs, but the dynamics of virtually the entire mangrove ecosystem.</p>

ABSTRACTNeotropical freshwater crabs are understudied but ecologically important consumers, with largely unknown responses to land use change and watershed urbanization. We used capture–mark–recapture methods to quantify Ptychophallus tumimanus (Pseudothelphusidae) freshwater crab populations across 20 headwater stream sites spanning a range of land uses, including forest reserves and an urban center in Monteverde, Costa Rica. We estimated site‐specific crab abundance and density using hierarchical Bayesian Markov chain Monte Carlo models and estimated the effects of potential covariates (elevation, water temperature, conductivity, % cobble, mean substrate size, channel width, depth, stream velocity, pH, % canopy cover) on crab abundances across sites. Crab abundances ranged from ~2 (95% CI: 0–9) to 143 (95% CI: 98–207) crabs across stream reaches. Densities ranged from 0.06 (95% CI: 0–0.25) to 5.21 (95% CI: 3.38–8.00) crabs/m of stream length. Probability of capture was estimated as 9.7% for a 30 min search time. Streams in undisturbed forested watersheds had higher crab abundances than sites near human development, where we found few to no crabs. Of the 10 predictor variables, conductivity and % cobble substrate best explained differences in P. tumimanus abundances across sites. Urban streams had low crab abundances and higher conductivities than forested streams, which suggests that this crab species may be sensitive to in‐stream effects of urbanization. This study demonstrates that capture–mark–recapture methods can be useful for estimating and understanding how crab abundances may vary across streams.

Crabs are among the most dominant macrofauna in mangrove ecosystems. Crabs play vital roles in maintaining the ecological functions of mangroves. However, there are limitations in assessing crab populations accurately. In previous studies crabs are caught or visually observed by researchers to obtain data on crab abundance and diversity. However, difficulties and biases arise due to complications in sighting crabs accurately and minimizing damage caused by intrusive methods used when attempts are made to capture crabs in situ. Therefore, more accurate observations, and less intrusive method should be implemented to identify crab populations. Two less destructive methods to investigate crab abundance were used in this study and compared to ascertain the efficiency of obtaining mangrove crab population data. Findings show that video recording method was more accurate compared with hand catch method in providing crab abundance data as hand catch method resulted in the under-sampling of crab abundance. This video recording method may be used: (1) to identify the abundance of crabs in a quadrat, (2) to provide crab population data as the quadrat is replicated in multiple sampling sites, and (3) to observe crab behaviour.

Abundance of Atlantic blue crab Callinectes sapidus is increasing in New England coastal estuaries which are well-documented blue crab habitat. Blue crab population and habitat were characterized in the anthropogenically altered Kickemuit River in Rhode Island, USA, to evaluate abundance, habitat use, and tolerances to anthropogenic impact. Crabs in dammed and undammed portions of the river were monitored for abundance, size, maturation, and sex from May to October 2023 using crab pots. Water quality and substrate data were collected to elucidate the relationships among crab population, seasonal environmental patterns, and habitat preference. It was hypothesized that benthic habitat characteristics would affect blue crab abundance, size, maturation, and sex between contrasting sites due to spatial segregation of sexes, ontogeny patterns, and anthropogenic and tidal alteration of this estuarine system caused by dams. Despite damming presenting a significant physical barrier, crab abundance was not considerably different north and south of the dam. Crabs were resilient to hypoxia and temperature extremes. The river is home to a resident male population and transient female population, following similar niche partitioning in the Chesapeake region. Salinity and community composition differences between sites suggest the dam limits tidal exchange north of the dam, effectively splitting the river into separate aquatic and estuarine ecosystems. These data suggest blue crab exhibits resilience that would make them successful in benthic ecosystems as populations continue expanding northward and occupy new areas with historical large-scale anthropogenic alteration.

Fiddler crab deposit-feeding and meiofaunal abundance in salt marsh habitats

<p>Abundant crab burrows in salt marshes can act as preferential water flow conduits for solute exchange by perforating muddy sediments, yet the impact of crab burrows on modulating carbon sequestration potential are not well understood. The field observations to assess how crab burrows drive carbon exchange over time scales of minutes to weeks along an intertidal marsh transect in North Inlet, South Carolina were conducted. The field analysis found that (1) continuous pore water exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients; (2) the burrow CO<sub>2</sub> concentrations is higher than on the marsh surface because of respiration. The average gas-phase CO<sub>2</sub> concentration in the crab burrows was approximately six times greater than ambient air. The lab analysis found that the concentrations of DIC and DOC in crab burrow porewater were lower than porewaters in the surrounding soil matrix. The porewater δ<sup>13</sup>C-DIC signatures in the crab burrows were heavier than those in the soil matrix, reflecting a mixture with seawater.</p><p>Crab burrows can influence carbon export by three pathways: gas phase CO<sub>2</sub> release from burrows caused by the irrigation of surface water, hydraulic gradient-driven porewater exchange (PEX) and concentration gradient-driven passive diffusion transport (PDT). Among these pathways, PEX showed the dominance of the crab burrow-induced carbon export, which is at least an order of magnitude higher than the others. Crab burrow-induced carbon export from the whole intertidal transect was calculated as the sum of CO<sub>2</sub> release and the dissolved carbon loss by PEX and PDT transport from the surrounding soil matrix to the crab burrow. The estimated C export was extrapolated to US East Coast salt marshes, which was nearly equivalent to riverine DIC flux, half of salt marsh DIC exports. These new insights underline the ecological roles of crab burrows in salt marsh carbon budgets, especially the importance of porewater exchange between crab burrows and the surrounding soil matrix.</p>

The large number of pollutants discharged into the aquatic environment may influence the physicochemical and biological qualities of the aquatic ecosystem. This study discloses the global quality of the surface waters and the effect of physicochemical variables on the abundance of the African freshwater crab Potamon algeriense inhabiting Zegzel watercourse, a mountain stream in the northeast of Morocco. Physicochemical variables including streamflow, water temperature, pH, dissolved oxygen, suspended matter, chloride, calcium, magnesium, ammonia, nitrite, nitrate, orthophosphate, and organic matter were evaluated monthly for one year (October 2017-September 2018). The evaluation of the physicochemical quality showed that the waters of all the stations studied are between the excellent and good quality classes concerning all the physicochemical variables and highlights also two variations with a tendency towards degradation, one spatial from upstream to downstream and the other seasonal from the wet to the dry period. The abundance of crabs was recorded to exhibit a positive correlation with dissolved oxygen, calcium (N = 44) (p < 0.01), and magnesium. However, a negative correlation has been noticed for streamflow (N = 1) (p < 0.01), water temperature, pH, suspended matter, chloride, ammonia, nitrite, nitrate, orthophosphate, and organic matter. The results obtained reveal that besides the biotic variables, the distribution of P. algeriense also depends on these specific environmental variables.

The present communication is aimed at assessing the molluscan diversity and distribution under the impact of some physico-chemical variables on them in Gho-Manhasan stream (a distributory of Chenab), Jammu (J & K). The study involved bottom sampling for 12 months (June, 2010 to July, 2011) from the banks of the stream and registered a total of 11 taxa of freshwater molluscs from class Gastropoda and Bivalvia representing 54.453 and 45.546% of the total population of molluscs. Numerically, Pissidium mitchelli (Bivalvia) was the most abundant taxa followed by Melanoides tuberculata (Gastropoda). Lymnea luteola, Bellamya bengalensis, Physella acuta, Gyraulus ladacensisshowed less frequent appearance whereas Lamellidens corrianus and Corbicula cashmeriensis were recorded as rare taxa. Analysis of the data revealed that the physico-chemistry of the stream exercised profound effect on the diversity and distribution of the malacofauna. Significant changes in molluscan assemblages were primarily due to changes in the stream water quality. Coefficient of correlation (r) between molluscs and physico-chemical parameters revealed significant relationship with bivalves. Key words: Physico-chemical profile, distributory, gastropoda, bivalvia, malacofauna and correlation.

Preliminary investigation in August, 2017 reported the presence of Lymnaea natalensis, Bellamya unicolor, Melanoides tuberculata and Bulinus globosus in order of increasing abundance and distribution in Warwade dam, Dutse, Jigawa State, Nigeria. A follow up study was carried out from April to October, 2019 to reveal further details on the abundance and distribution of freshwater snails in relation to some physiochemical factors of the dam. Four sampling sites; human activity, vegetation cover, lentic and lotic were selected for the study along the bank of the dam. Freshwater snails were collected using long handled scoop net with mesh 0.2mm complemented by hand picking methods in the four sampling sites. Water samples from the sampling sites were analyzed in the laboratory using standard procedures. A total of 2,027 of freshwater snails belonging to ten species were identified. Bulinus globosus 12(0.6%) and Lymnaea natalensis 12(0.6%) had the lowest abundance and distribution while Melanoides tuberculata 1553(76.6%) had the highest. Snail abundance was highest in site characterized by human activities (670) followed by vegetation (482), lotic (442) and lentic (433) waters. Most of the physico-chemical factors measured appeared to favour the growth and survival of fresh water snails. pH (p = 0.01), water current (p = &lt;0.01) and magnesium ion concentration (p = &lt; 0.01) varied significantly across the four sites. Only calcium ion concentration was significantly associated with snail abundance (p = 0.04). Snail abundance showed weak positive relationship with water temperature, color, turbidity and concentration of magnesium ion. The dam habours about ten species of freshwater snails in different abundance and distribution with M. tuberculata being the most abundant throughout the period of investigation. The dominance of M. tuberculata over other species particularly those of medical and veterinary importance could have positive implication for their control in the dam.

Ovia South-West Local Government Area of Edo State in Nigeria is endemic for urogenital schistosomiasis. However, there are no reports of snail hosts of the disease in any community within the locality. Ayear-long freshwater snail survey was conducted in seven localities (Aden, Igbogor, Ikoha, Okopon, Okponha, Siluko, and Ugbogui) in Ovia South-West to determine the snail diversity and identify species that are intermediate hosts for schistosomiasis. A total of 468 snails representing species in four genera (Bulinus, Lanistes, Melanoides, and Radix) were collected. Based on the morphological appearances of shells, five snails, B. forskalii, B. globosus, B. truncatus, L. varicus, and M. tuberculata were identified at the species level. The snail Radix could not be identified at the species level. Melanoides tuberculata was the most abundant snail, and the only species identified at all sampled locations (n = 284, 60.68%). The next most abundant species was Radix sp. (n = 106, 22.65%), followed by B. globosus(n = 42, 8.97%). The least abundant snail was B. truncatus (n = 5, 1.07%). In Siluko, all species of snails were present, and only M. tuberculata was present in Ugbogui. In both the dry and wet seasons, all snail species were present (wet, n = 297) (dry, n = 171). Similar snail diversity occurred at each sampling location in both seasons, with more individuals of each species collected during the wet season. The only infected snail was a single Radix sp. in Siluko that shed the brevifurcate-apharyngeate distome furcocercariae. All the other snail species were uninfected.