Genetic differentiation and speciation in the estuarine isopodsCyathura polita andCyathura burbancki

We investigated the electrophoretically detectable variation at nine protein loci in seven populations ofCyathura polita and one population of the closely relatedC. burbancki from the Atlantic coast of North America. The average proportion of the genome heterozygous per individual \((\bar H)\) inC. polita ranges from about 0% in New Brunswick and Massachusetts to 9.3% in a Georgia population. \(\bar H\) in the population ofC. burbancki was 2.6%. On the basis of our results,C. polita can be separated into two geographic forms,northern (New Brunswick, Massachusetts, and Maryland) andsouthern (Georgia and Florida). The genetic distances between the southern and northernC. polita are similar to the genetic distances reported between species of several other invertebrate groups. Our genetic data support the idea that the northern and southern forms ofC. polita andC. burbancki comprise a triad of species that diverged from each other 2–3 million years ago. Since we know nothing about the genetic structure of populations ofCyathura between Georgia and Maryland, this interpretation must be considered preliminary.     

Seasonal and spatial distribution of the Carolina marsh clamPolymesoda caroliniana (Bosc) in a Mississippi tidal marsh

The abundance of the marsh clamPolymesoda caroliniana at four stations on a Mississippi tidal marsh was followed from February 1977 through April 1978. The stations varied with respect to inundation time and vegetative cover. The largest population was found in an area flooded 12% of the year (136 individuals per m²) and in an area flooded 3.2% of the year (126 individuals per m²). However, the latter area contained no large clams (>25.5 mm). Smaller populations were found in the upper reaches of a tidal creek draining the marsh (68 individuals per m²) and in an area flooded 5.8% of the year (73 individuals per m²). The average abundance of the clams on the marsh was 100 per m². At least three recruitment periods were apparent. In laboratory studies, clams larger than 15 mm survived longer than 30 days without water. Clams smaller than 12 mm survived less than 6 days without water. Several decapods (Callinectes sapidus, Uca longisignalis, andSesarma reticulatum) consumed clams smaller than 10 mm but were unable to eat larger clams under laboratory conditions.Polymesoda caroliniana may be important to the functioning of the marsh by removing large quantities of particulate matter from flooding water.     

Changes in the distribution of seagrass species along Florida's Central Gulf Coast: Iverson and Bittaker revisited

A broad-scale survey of seagrass species composition and distribution along Florida's central Gulf Coast (known as the Big Bend region) was conducted in the summer of 2000 to address growing concerns over the potential effects of increased nutrient loading from adjacent coastal rivers. Iverson and Bittaker (1986) originally surveyed seagrass distribution in this region between 1974–1980. We revisited 188 stations from the original survey, recording the presence or absence of all seagrass species. Although factors such as accuracy of station relocation, differences in sampling effort among studies, and length of time between surveys preclude statistical comparisons, several interesting patterns emerged. While the total number of stations occupied by the three most common seagrass species,Thalassia testudinum, Syringodium filiforme, andHalodule wrightii, was similar between the two time periods, we observed a change in the number of records of each species as well as changes in distribution with depth.T. testudinum andHalophila engelmanni occurrence declined in the deepest areas of the region, while the number of stations occupied byS. filiforme andH. wrightii increased in nearby areas. We observed several localized areas of seagrass loss, frequently associated with the mouths of coastal rivers. These results suggest that increased nutrient loading to coastal rivers that discharge into the Big Bend area may be affecting seagrasses by increasing phytoplankton abundance in the water column, thus changing water clarity characteristics of the region.     

Seasonal abundance of aquatic diptera in two oligohaline tidal marshes in Mississippi

Nineteen species of Diptera (16 in each zone) from seven families were found in monthly collections (June 1979 through May 1980) in two Mississippi marsh plant zones dominated byJuncus roemerianus Scheele andSpartina cynosuroides (L.) Roth, respectively. TheJuncus zone was dominated by a species ofPalpomyia-Bezzia complex, two species ofBezzia (Ceratopogonidae),Paratendipes sp.,Limnophyes sp. (Chironomidae) andThinophilus frontalis (Dolichopodidae).Palpomyia-Bezzia sp.,Culicoides hollensis, Ormosia sp. (Tipulidae) andPelastoneurus abbreviatus (Dolichopodidae) dominated theSpartina zone. Total inset density and species richness were lowest in June and July in both zones, corresponding to a pulse of adult emergence. Overall, total density was significantly higher in theSpartina zone with mean values of 165 and 245 insects m^?2 for the {Juncus} and {Spartina} zones, respectively. In addition to total density, differences in abundance were apparent for a number of species between marsh zones. Patterns could be attributed to species-specific habitat preference., predation by aquatic predators, and/or by interactions of the infauna themselves.     

Ecotopic and ontogenetic trophic variation in mojarras (Pisces: Gerreidae)

Seven species of mojarras (Gerreidae) were collected from three stations in the Indian River lagoon near the Sebastian Inlet, Florida over an eighteen month period to study spatial and temporal variations in occurrence and feeding habits. Two major habitats common to this area were represented in the study; a sandy beach and a seagrass flat. Of the 2,899 gerreids collected,Eucinostomus gula represented 67% and was found primarily in theHalodule seagrass site. The next most abundant species,E. argenteus (11%),E. jonesi andE. lefroyi (each 10%), were found most frequently at the inlet site.Diapterus auratus, E. melanopterus, andGerres cinereus were captured infrequently and were considered incidental. Prey items found included amphipods, bivalves, copepods, fish eggs, foraminifera, isopods, nematodes, ostracods, polychaetes and crustacean parts. Significant amounts of sand and unidentifiable amorphous debris were also included in the gut content analysis. Little interspecific variation in diet was found among the four major species examined. The general shift seen from a diet composed primarily of copepods and other crustaceans to one of polychaetes suggests an intraspecific ontogenetic progression. Spatial variations in diet composition were also indicated between the two major habitats. Feeding analyses showed gerreids to be diurnal feeders with the greatest consumption (total gut content weight) occurring at dusk.     

Long-term pCO2 dynamics in rivers in the Chesapeake Bay watershed

Streams and rivers are major exporters of C and other dissolved materials from watersheds to coastal waters. In streams and rivers, substantial amounts of terrigenous organic C is metabolized and degassed as CO₂ to the atmosphere. A long-term evaluation of CO₂ dynamics in streams is essential for understanding factors controlling CO₂ dynamics in streams in response to changes in climate and land-use. Long-term changes in the partial pressure of CO₂ (pCO₂) were computed in the Anacostia River and the lower Potomac River in the Chesapeake Bay watershed. Long-term estimates were made using routine monitoring data of pH, total alkalinity, and dissolved nutrients from 1985 to 2006 at 14 stations. Longitudinal variability in pCO₂ dynamics was also investigated along these rivers downstream of the urban Washington D.C. metropolitan area. Both rivers were supersaturated with CO₂ with respect to atmospheric CO₂ levels (392 μatm) and the highly urbanized Anacostia waters (202–9694 μatm) were more supersaturated than the Potomac waters (557–3800 μatm). Long-term variability in pCO₂ values may be due to changes in river metabolism and organic matter and nutrient loadings. Both rivers exchange significant amounts of CO₂ with the atmosphere (i.e., Anacostia at 0.2–72 mmol m⁻² d⁻¹ and Potomac at 0.12–24 mmol m⁻² d⁻¹), implying that waterways receiving organic matter and nutrient subsidies from urbanized landscapes have the potential to increase river metabolism and atmospheric CO₂ fluxes along the freshwater–estuarine continuum.     

Zooplankton distribution and abundance in the Sacramento-San Joaquin delta in relation to certain environmental factors

The dominant members of the freshwater zooplankton in the Sacramento-San Joaquin delta were those typical of temperate zone rivers—Bosmina andCyclops among the crustaceans andKeratella, Polyarthra, Trichocerca andSynchaeta among the rotifers. The estuarine or brackish component of the plankton was represented by the copepodEurytemora affinis and the rotiferSynchaeta bicornis. Abundace of freshwater zooplankton was highest in the San Joaquin River near Stockton, the region with the highest chlorophylla concentrations and highest temperatures. This was also the region least affected by water project operations, which alter the normal river flow patterns and bring large volumes of zooplankton-deficient Sacramento River water into the San Joaquin River and south delta chanels. Over a seven-year period, abundance of most zooplankton genera was positively correlated with chlorophylla concentrations and temperature but not with net flow velocity. OnlyBosmina had a significant and negative correlation with abundance of a predacious shrimp,Neomysis mercedis. Extreme salinity intrusion in 1977 reduced freshwater zooplankton abundance throughout most of the delta to seven-year lows. All zooplankton groups showed a long-term abundance decline from 1972 to 1978. In the cases of rotifers and copepods, this deciline was significantly correlated with a decline in chlorophylla.     

Composition and genesis of the organic matter in the bottom sediments of the East Siberian Sea

The chemical composition of organic matter (C_org, N_org, δ¹³C, δ¹⁵N, and n-alkanes) was studied in the top layer of bottom sediments of the East Siberian Sea. Possible ways were proposed to estimate the amount of the terrigenous component in their organic matter (OM). The fraction of terrigenous OM estimated by the combined use of genetic indicators varied from 15% in the eastern part of the sea, near the Long Strait, to 95% in the estuaries of the Indigirka and Kolyma rivers, averaging 62% over the sea area.     

Effects of stormwater nutrient discharges on eutrophication processes in nearshore waters of the Florida keys

Rainfall events cause episodic discharges of groundwaters contaminated with septic tank effluent into nearshore waters of the Florida keys, enhancing eutrophication in sensitive coral reef communities. Our study characterized the effects of stormwater discharges by continuously (30-min intervals) measuring salinity, temperature, tidal stage, and dissolved oxygen (DO) along an offshore eutrophication gradient prior to and following heavy rainfall at the beginning of the 1992 rainy season. The gradient included stations at a developed canal system (PP) on Big Pine Key, a seagrass meadow in a tidal channel (PC), a nearshore patch reef (PR), a bank reef at Looe Key National Marine Sanctuary (LK), and a blue water station (BW) approximately 9 km off of Big PIne Key. Water samples were collected at weekly intervals during this period to determine concentrations of total nitrogen (TN), ammonium (NH₄ ⁺), nitrate plus nitrite NO₃ ^? plus NO₂ ^?), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), and chlorophyll a (chl a). Decreased salinity immediately followed the first major rainfall at Big Pine Key, which was followed by anoxia (DO <0.1 mg I^?1), high concentrations of NH₄ ⁺ (≈24 μM), TDP (≈1.5 μM), and chl a (≈20 μg I^?1). Maximum concentration of TDP (≈0.30 μM) also followed the initial rainfall at the PC, PR, and LK stations. In contrast, NH₄ ⁺ (≈4.0 μM) and chl a (0.45 μg I^?1) lagged the rain event by 1–3 wk, depending on distance from shore. The highest and most variable concentrations of NH₄ ⁺, TDP, and chl a occurred at PP, and all nutrient parameters correlated positively with rainfall. DO at all stations was positively correlated with tide and salinity and the lowest values occurred during low tide and low salinity (high rainfall) periods. Hypoxia (DO <2.5 mg I^?1) was observed at all stations follwing the stormwater discharges, including the offshore bank reef station LK. Our study demonstrated that high frequency (daily) sampling is necessary to track the effects of episodic rainfall events on water quality and that such effects can be detected at considerable distances (12 km) from shore. The low levels of DO and high levels of nutrients and chl a in coastal waters of the Florida Keys demand that special precautions be exercised in the treatment and discharge of wastewaters and land-based runoff in order to preserve sensitive coral reef communities.     

Diel variation in immigration of fishes and decapod crustaceans to artificial seagrass habitat

Fish and decapod entry into small (1.5 m²) artificial seagrass habitats positioned on an open sand area in a New Jersey estuary was examined to determine if immigration varied between day and night. To encounter the structured habitats, colonizers had to cross an expanse of bare sand, with its presumably higher predation risk. Contrasts in abundance in the artificial seagrass plots between dawn and dusk indicated higher nighttime immigration for four species, including the fishesFundulus heteroclitus andMyoxocephalus aenaeus, and the caridean shrimpsPalaemonetes vulgaris andHippolyte pleuracanthus. Size-frequency distributions of colonizers varied between day and night for two fish species,Menidia menidia andSyngnathus fuscus, with a greater proportion of smaller individuals immigrating to the artificial seagrass at night.Callinectes sapidus also displayed a diel contrast in size distribution but, for this species, proportionately more small individuals colonized the plots during the day. We suggest that diel variability in predation risk and/or diel patterns in motor activity may be responsible for these patterns in immigration.     

A comparison of phytoplankton assemblages and environmental relationships in three estuarine rivers of the lower Chesapeake Bay

A 16-month data set of phytoplankton assemblages and environmental parameters were studied in the lower James, York (-Pamunkey), and Rappahannock rivers using several exploratory statistical approaches. Based on species composition and river station relationships, three site groups were established and subsequently identified as predominantly tidal fresh, oligo-mesohaline, and mesohaline sites. Phytoplankton assemblages within these rivers were influenced and subsequently augmented by the onset of the spring freshet which was different in 1986 and 1987. Five temporal assemblages of phytoplankters were also identified and designated into seasonal groupings of spring 1986, summer-fall, summer-winter, fall-winter, and winter-spring 1987. Discriminant analysis (MANOVA) evaluations were made for water quality parameters to site and seasonal phytoplankton assemblages and these relationships are discussed. Moving downstream along an oligohaline-mesohaline gradient, the nitrogen and phosphorus levels decreased and the phytoplankton composition was more similar at several corresponding site locations in the different rivers than at stations relatively close to each other in the same river. Within these data sets approximately 58% of the explained variance was associated with site (spatial) effects, 30% with temporal effects, and 12% with site-temporal interactions. A transition from dominant bloom-producing freshwater diatoms to estuarine species occurs from the tidally influenced freshwater zone downstream. This change may be rapid as the decline ofSkeletonema potamos, or more gradual, as withCyclotella striata andCyclotella meneghiania. These are replaced downstream bySkeletonema costatum, Cyclotella caspia, andLeptocylindrus minimus as dominant species.     

Trophic ecology of the dominant fishes in Elkhorn Slough,California, 1974–1980

Food habits of the dominant fishes collected from 1974 to 1980 at eight locations in Elkhorn Slough, California, and the adjacent ocean were investigated. Epifaunal crustacea was the major prey group identified from stomach contents of more than 2,000 fishes, followed by epifaunal and infaunal worms, and molluscs. Overall, 18 fish species consumed 263 different prey taxa, ranging from 10 taxa to 125 taxa per fish species and including 99 crustacean, 56 polychaete, and 39 molluscan taxa. Mean prey richness was greatest at stations near the ocean and lowest at inshore stations. Detailed dietary data for all prey taxa were summarized as trophic spectra for each fish species. Trophic spectra represented functional groups of prey and were used for comparisons of dietary similarity. Cluster analyses, based on trophic spectra, resulted in four feeding guilds of fishes. Of 18 fish species, seven (Amphistichus argenteus, Leptocottus armatus, Embiotoca jacksoni, Clevelandia ios, Gillichthys mirabilis, Cymatogaster aggregata, andCitharichthys stimaeus) fed principally on epifaunal crustacea. Four species (Pleuronectes vetulus, Platichthys stellatus, Phanerodon furcatus, andMyliobatus californica) consumed mostly molluscs and infaunal worms. Two species (Psettichthys melanostictus andTriakis semifasciata) fed on mobile crustacea, and five species (Hyperprosopon anale, Engraulis mordax, Clupea pallasi, Atherinopsis californiensis, andAtherinops affinis) fed largely on zooplankton and plant material. Our results suggest that high food availability enhances the nursery function of imshore habitats, and emphasize the importance of invertebrate prey populations and the indirect linkage of plant production to the ichthyofaunal assemblarly marine immigrant species that are likely ‘estuarine dependent’.     

A study on cosmogenic nuclides in the Jilin meteorite and its irradiation history

44 samples of the Jilin meteorite were analyzed by various laboratories for spallogenic, radiogenic, and trapped rare gases. A non-uniform distribution has been found for the rare gases of different origins. There have been found correlations among the spallogenic rare gases, with apparent depth effects. 43 samples were analyzed for their cosmogenic radionuclides⁶⁰Co,²⁶Al,³⁶Cl,⁴⁰K,⁵³Mn,⁵⁴Mn, and²²Na. Correlations have been found between⁶⁰Co and⁵³Mn and between⁶⁰Co and²¹Ne. The Jilin meteorite has a complex history of cosmic-ray irradiation. According to the two-stage model and the other fragments have a burying depth two stages,T ₁=11 m.y. andT ₂=0.3 m.y. The burying depth of all samples in the parent body can be obtained by the content of²¹Ne. Jilin meteorite No. 1 is located 20–142 cm from the surface, No. 4 ranges from 106–134 cm, and the other fragments have a burying depth between 15 and 150 cm. The equation of the reference plane for the surface of the 1-stage Jilin meteoroid is 0.24x+0.81y+0.53z+0.5=0. Use can also be made of⁶⁰Co to determine the burying depth of all samples in the 2-stage Jilin meteoroid (under a geometry of 4π), and further to restore the preatmospheric form and size of the parent body. During its atmospheric passage, the Jilin had an ablation rate of ca. 30%. On the basis of the two-stage model and the concentration of²¹Ne and⁶⁰Co in the samples, we propose a scheme to restore the relative position of all samples in the parent body. 12 sample-distribution regions can be sketched out. Also can be restored the relative position of all the samples in the parent body during the two stages.     

Long-term declines in two apex predators,bull sharks (<Emphasis Type="Italic">Carcharhinus leucas</Emphasis>) and alligator gar (<Emphasis Type="Italic">Atractosteus spatula</Emphasis>), in Lake Pontchartrain,an oligohaline estuary in southeastern Louisiana

We analyzed historic and current fishery independent data to determine if the abundance of two apex predators, bull sharks (Carcharhinus leucas) and alligator gar (Atractosteus spatula), in Lake Pontchartrain had changed significantly over the last half century. Lake Pontchartrain is an environmentally degraded oligohaline estuary in southeastern Louisiana that has experienced considerable changes in fish assemblage composition over this interval. Using gillnet, beach seine, and trawl data collected during three time periods (1953–1955, 1977–1978, and 1996–2005), we analyzed trends in abundance forC. leucas andA. spatula using generalized linear models with a negative binomial error structure and a log link. Lake Pontchartrain data were divided into four spatial locations (northwest, northeast, southwest, southeast) since each region represents a unique combination of anthropogenic and natural influences that could affect catches. For each species and gear type, we produced log-likelihood profiles for the instantaneous rate of change in relative abundance through time. Raw catches were generally lower for both species in the later surveys.C. leucas were not captured in beach seines since the 1950s andA. spatula were rarely captured in trawls or seines since the 1970s. Likelihood profiles of changes in abundance forC. leucas andA. spatula showed very large declines in both species since 1953.C. leucas declined by 98.6% (95% CI: 73.4–99.9%) in gillnets and became functionally extirpated in beach seines with a decline of 99.9% (95% CI: 23–99.9%). Among all gears,C. leucas declined by the same rate as in gillnets. The decline inA. spatula was also large with a decrease of 98.6% (95% CI: 73.4–99.9%) in beach seines and a decline of 99.2% (95% CI: 54.8–99.9%) in trawls since 1953. Catches ofA. spatula in gillnets did not show a significant change over the study period. The continued decline of these two apex predators could seriously affect efforts to restore this degraded estuarine ecosystem.     

The effect of migratory shorebirds on the benthic species of three southwestern Atlantic Argentinean estuaries

We experimentally evaluate the effect of migratory shorebirds on the benthic fauna of three southwestern Atlantic Argentinean stop-over and wintering sites: Bahía Samborombon (35°30′–36°22′S, 57°23′W), Mar Chiquita coastal lagoon (37°40′S, 57°26′W), and Bahía Blanca (38°48′–39°25′S, 50°–62°25′W). The experiments consisted of exclusion ceilings and controls (both 1 m²), with 10 replicates each and aligned at the same tidal level. During December 1994–May 1995 these experiments were done, twice in Bahía Samborombon, twice at two sites (Sotelo and Celpa) 4 km apart in Mar Chiquita coastal lagoon, and three times in Bahía Blanca. Three polychaete species (Laeonereis acuta, Nephtys fluviatilis, andHeteromastus similis) were found in similar densities in both areas of the Mar Chiquita lagoon.L. acuta andN. fluviatilis were affected in Sotelo, but there was no treatment effect in Celpa. In Sotelo the most abundant shorebirds were Hudsonian godwit (Limosa haemastica), American golden plover (Pluvialis dominica), and White-rumped sandpiper (Calidris fuscicollis), and in Celpa the most abundant shorebirds were White-rumped sandpiper, two-banded plower (Charadrius falklandicus), and lesser yellowlegs (Tringa flavipes). The largest polychaete densities were in the upper sediment layer (0–6 cm), which was also the most affected layer. Three polychaete species were found in Bahía Samborombon (L. acuta, H. similis, andNeanthes succinea), but onlyH. similis showed a treatment effect. The most abundant shorebirds in Samborombon were white-rumped sandpiper and two-banded plover. No effect was detected in Bahía Blanca, where the most abundant shorebirds were white-rumped sandpiper and American golden plover. In all cases, the species affected by shorebirds were the most abundant species. From the two sites of Mar Chiquita, there was a treatment effect only in Sotelo, which was also the area with higher shorebirds counts. However, much lower densities observed in Samborombon (similar to the unaffected area of Mar Chiquita) also produced a significant decrease in infaunal abundance. These evidences also suggest that just selection of study sites may produce different views of the interaction between shorebirds and benthic species.     

Oligohaline benthic invertebrate communities at two Chesapeake Bay power plants

Benthic invertebrate populations at the Surry power plant on James River, Virginia and the C. P. Crane power plant on Saltpeter Creek, Maryland exhibited large spatial and temporal variations. At C. P. Crane, where the cooling water is pumped between two tidal creeks, populations in the receiving creek exhibited five response patterns: 1) mitigation of a winter dieoff (Rangia cuneata, a brackish water clam), 2) acceleration of growth or development (R. cuneata; Scolecolepides viridis, a polychaete;Leptocheirus plumulosus, an amphipod; Tubificidae; andCoelotanypus sp., a dipteran), 3) importantion of larvae from the source water creek (S. viridis andCoelotanypus sp.), 4) extension of creek-dwelling species into the adjacent river (Coelotanypus sp. and other dipterans), and 5) increased severity of late summer population depressions (S. viridis andL. plumulosus). At Surry, where the cooling was no confined creek system at the discharge, and effects were less pronounced. The community in the Surry discharge zone resembled the community at downriver stations more closely than at upriver reference stations. TheL. plumulosus population near the Surry discharge was depressed, while theMytilopsis leucophaeta population was enhanced. No major ecological damage was attributed to either power plant, due in part to the resilience of estuarine endemic populations, but the unique features exhibited by each of the two sites support the argument that oligohaline estuarine zones should not be designateda priori for unregulated industrial development.     

Variations of Pb in a mine-impacted tropical river,Taxco, Mexico: Use of geochemical,isotopic and statistical tools

The potential environmental threat from Pb in Mexican rivers impacted by historic mining activities was studied using geochemical, isotopic and statistical methods. Lead geochemical fractionation and factor analysis of fractionated and total Pb indicate that anthropogenic sources have contributed significantly to Pb concentrations, while natural sources have contributed only small amounts. The analyses also indicate that two main processes are controlling the total Pb variation throughout the year in both rivers: erosion with discharge processes, and proportional dilution related to differences in grain-size distribution processes. Bio-available Pb in riverbed sediments was greater than 50% in 80% of the sampling stations indicating a high potential environmental risk, according to the risk assessment criteria (RAC). Nevertheless, based on the environmental chemistry of Pb and on multivariate statistical analysis, these criteria did not apply in this particular case. Significant differences (p < 0.05) in total Pb concentrations (from 50 to 5820 mg kg⁻¹) and in the geochemical fractionation were observed as a function of seasonality and location along the river flow path. In the Cacalotenango and Taxco rivers, the highest concentrations of total Pb were found at stations close to tailings during the rainy and post-rainy seasons. The geochemistry of Pb was mainly controlled, during the dry and post-rainy seasons by the organic matter and carbonate content, and in the rainy season by hydrological conditions (e.g., the increase in river flux), hydrological basin erosion, and the suspended solids concentration. Isotopic analyses of the ²¹⁰Pb/²¹⁴Pb ratio showed three processes in the Cacalotenango and Taxco rivers. First, the accumulation of atmospheric excess ²¹⁰Pb, favoured during calmer hydrodynamic conditions in the river basin commonly during dry periods, is recorded by a ²¹⁰Pb/²¹⁴Pb ratio of >1. In the case of the Cacalotenango river, ²¹⁰Pb did show preferential accumulation in sediments. Second, a ²¹⁰Pb/²¹⁴Pb ratio of <1 in some samples might be indicating (a) the presence of eroded material from weathered tailings with similar ²¹⁰Pb depletion (probably by secular disequilibrium caused by weathering or mining processes, which was observed in both rivers), or (b) preferential transport of Pb and sediments during high energy events (e.g., flow increase, as is the case of Taxco river). Third, no significant changes in the ²¹⁰Pb/²¹⁴Pb ratio might be reflecting a situation where mining material is not entering the system, or where hydrodynamic changes throughout the year of equal magnitude allow the system to reach a new equilibrium for the ²¹⁰Pb/²¹⁴Pb ratio. Finally, based on these results it is recommended that inhabitants of the studied area avoid using water from the Cacalotenango river in the rainy and post-rainy seasons, and to take precautions for its use in the dry season, such as allowing suspended material to settle before use, and that they should avoid use of Taxco river water at all times.     

Environmental effects of river sand mining: a case from the river catchments of Vembanad lake,Southwest coast of India

Rivers in the southwest coast of India are under immense pressure due to various kinds of human activities among which indiscriminate extraction of construction grade sand is the most disastrous one. The situation is rather alarming in the rivers draining the Vembanad lake catchments as the area hosts one of the fast developing urban-cum-industrial centre, the Kochi city, otherwise called the Queen of Arabian Sea. The Vembanad lake catchments are drained by seven rivers whose length varies between 78 and 244 km and catchment area between 847 and 5,398 km². On an average, 11.73 million ty⁻¹ of sand and gravel are being extracted from the active channels and 0.414 million ty⁻¹ of sand from the river floodplains. The quantity of instream mining is about 40 times the higher than the sand input estimated in the gauging stations. As a result of indiscriminate sand mining, the riverbed in the storage zone is getting lowered at a rate of 7–15 cm y⁻¹ over the past two decades. This, in turn, imposes severe damages to the physical and biological environments of these river systems. The present paper deals with the environmental effects of indiscriminate sand mining from the small catchment rivers in the southwest coast of India, taking the case of the rivers draining the Vembanad lake catchments as an example.     

Postsettlement patterns of habitat use by sciaenid fishes in subtropical seagrass meadows

Spatial and temporal patterns of distribution and abundance were examined for postsettlement sciaenids collected from seagrass meadows in the Aransas Estuary, Texas. Overall, 5443 sciaenid larvae and early juveniles were identified from biweekly epibenthic sled collections taken from August 1994 to August 1995. Eight species were present in seagrass meadows, with five accounting for over 99.9% of sciaenids collected: silver perch (Bairdiella chrysoura), spotted seatrout (Cynoscion nebulosus), spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus), and red drum (Sciaenops ocellatus). Settlement to seagrass meadows was partitioned temporally with little overlap among the five species. Postsettlers from inshore spawners (B. chrysoura, C. nebulosus, S. ocellatus) inhabited seagrass meadows during the spring and summer, while individuals from offshore spawners (L. xanthurus, M. undulatus) were present in the late fall and winter. Densities ofB. chrysoura, C. nebulosus, S. ocellatus were highest for small individuals (4–8 mm SL) and these taxa remained in seagrass sites through the early juvenile stage. Conversely,L. xanthurus andM. undulatus maintained longer pelagic periods and generally entered seagrass meadows at larger sizes (10–14 mm SL). Moreover, these taxa were only temporary residents of selected seagrass meadows, apparently migrating to alternative habitats shortly after arrival. During peak settlement, mean and maximum densities among species ranged from 0.1 m^?2 to 0.8 m^?2 and 0.7 m^?2 to 23.8 m^?2, respectively. Density and mean size of possettlement sciaenids differed significantly between seagrass species (Halodule wrightii, Thalassia testudinum) and among sites within the estuary.     

Flushing times for the Providence River based on tracer experiments

The flushing time of the Providence River was estimated using three different data sets and three different methodologies. Dye concentrations were measured following instantaneous dye releases during wet weather experiments performed by the Narragansett Bay Project between October 1988 and June 1989. These data were analyzed to obtain flushing time estimates. Salinity measurements collected during the Sinbadd (Sampling In Narragansett Bay All During the Day) cruises, Spray (Sampling the Providence River All Year) cruises and wet weather experiments were used with the fraction of fresh water method and box model to calculate flushing time. The Sinbadd cruises performed 4 seasonal surveys at 22 stations in Narragansett Bay during 1986 to obtain a view of the whole Narragansett Bay with respect to the concentrations of nutrients and trace metals. The Spray cruises collected data in the Providence River at 10 stations to determine the relationship of nutrients and trace metals concentrations in the Seekonk and Providence rivers as a function of point source inputs. Based on the flushing time estimates, an exponential relationship between freshwater inflow and flushing time was developed (correlation coefficient of 0.826). The flushing time ranged from 0.8 d at high (90 m³ m^?1) freshwater inflows to 4.4 d at low (20 m³ s^?1) freshwater inflows. The average flushing time of the Providence River was estimated as 2.5 d for the mean freshwater inflow of 42.3 m³ s^?1.