Downriver Sites Research Articles

Growth of Pacific staghorn sculpin (Leptocottus armatus) is reduced at contaminated sites in the Lower Duwamish River, Washington

The Lower Duwamish River is a highly industrialized waterway flowing into the densely urbanized Puget Sound waterfront of Seattle, Washington, USA. The river has been profoundly altered from its natural state following more than a century of channelization, recurrent dredging, shoreline armoring, and pollution discharges. As part of a Natural Resource Damage Assessment addressing historical pollution at three designated Superfund sites (i.e., the assessment area), juvenile Pacific staghorn sculpin (Leptocottus armatus) were sampled throughout the lower river in order to evaluate injury from exposure to polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDTs), and butyltins (BTs). Sculpin live in close association with the river sediments within and upriver of the assessment area. Fish were collected for analysis of contaminant concentrations in composited whole bodies and stomach contents, as well as individual fish health metrics including daily somatic growth rates measured from otoliths. Sediment contaminant concentrations were also measured at sites near to fishing locations. Fish growth rates varied from 0.65 to 1.05 mm/day, and were significantly lower at unremediated downriver sites compared to upriver and remediated locations. Sculpin growth rates were negatively correlated with concentrations of PCBs in fish bodies, PAHs in stomach contents, as well as PCBs, DDTs and PAHs in sediment. Mixed effects models for whole-body and stomach content contaminants showed positive correlations between growth rate and water temperature. Temperature was not a significant confounding variable for the relationship between growth rate and sediment contaminants. Overall, these results show that juvenile sculpin are harmed by contaminant exposure in the Lower Duwamish River. Furthermore, this study demonstrates the utility of using paired biological and chemical indicators of pollutant-induced injury in a resident fish to inform a complex Natural Resource Damage Assessment and associated restoration efforts.

Survival and growth performance of Crassostrea virginica along an estuarine gradient

In Eastern Canada, an increasing number of pests, predators and pathogens in downriver areas (close to the ocean) have led the aquaculture industry to consider growing oysters in upriver areas (close to the river source). In this study, oyster growth performance was compared between downriver and upriver environments by means of stock transfer experiments within the Richibucto estuary. In May 2009, seed oysters (~26 mm shell height) originating from two downriver sites (salinity ~20–30 ‰) were transferred upriver (salinity ~5–20 ‰). Follow-up measurements in October 2009 revealed that the seed transferred upriver grew and survived as well as seed that remained downriver, while the mortality rates of adult oysters (shell height ~66 mm) were lower at the upriver site. Meat content was unaffected in adult oysters transferred upriver. However, there were indications that the upriver environment promoted shell growth (mm) in adult oysters. Oysters transferred upriver had a gain in shell height (LSM ± SE) over the oysters that remained downriver (2.7 ± 0.5 vs. 1.8 ± 0.5 mm) and in shell width [Median (95 % CI); 2.8 mm (1.9, 3.6) vs. 1.0 mm (0.3, 1.2)]. Therefore, the holding of adult oysters upriver during the spring–summer period confers productivity advantages on top of protection from diseases and predation. By contrast, productivity losses were recorded when relocating adult oysters originating from the upriver environment. More specifically, final organic meat content were approximately 35 % less in adult oysters transferred downriver compared to those that remained upriver (0.48 ± 0.04 vs. 0.74 ± 0.04 g). Results suggest that transfers along the river impact physiological processes such as gametogenesis and shell formation in adult oysters.

Stress response ofCrassostrea virginica(Gmelin, 1791) oysters following a reciprocal transfer between upriver and downriver sites

This study was designed to assess the physiological impact of transferring oysters between upriver and downriver aquaculture sites, a common practice in North America that is primarily aimed at reducing disease infections and predation on cultured stocks. In May 2009, oysters (Crassostrea virginica) were reciprocally transferred between an upriver and a downriver site in the Richibucto estuary in eastern Canada. Mortality, tissue and cellular stress responses were subsequently evaluated in August and October 2009. Overall, oyster mortality remained low (~5%) throughout the 5-month study period with no significant difference between sites or oyster sources. However, by October oysters reared at the upriver site, regardless of their origin, had significantly higher levels of lysosomal membrane destabilization (63.6%, SE = 1.9) and digestive tubule atrophy (33.3–42.4%, SE = 3.6) than oysters reared at the downriver site (47.5%, SE = 1.8; 15.6–19.1%, SE = 3.8 respectively). They also exhibited a greater salinity differential between their mantle/haemolymph fluids and the ambient seawater, possibly indicating more restricted exchange with the environment. In general, the transfer of upriver oysters to a downriver site had a positive impact, i.e. lower levels of lysosomal destabilization and tubule atrophy, whereas transfer of downriver oysters upriver had the opposite effect. These results suggest that upriver environmental conditions negatively impact cellular and tissue integrity in oysters without leading to mortality during the summer–autumn period.

River discharge, land use change, and surface water quality in the Xiangjiang River, China

AbstractTo compare the impacts of river discharge on the surface water quality of the Xiangjiang River in China, 12 surface water quality parameters recorded at 31 sampling sites from January 1998 to December 2008 along the river and its main tributaries were analyzed. Significantly higher concentrations of total nitrogen, ammoniacal nitrogen, and total phosphorus, and biochemical oxygen demand were observed during low‐flow periods than during high‐flow periods, implying a higher risk to local residents drinking untreated water during low‐flow periods. Pollution indexes, including the inorganic pollution index and integrated pollution index (IPI), were negatively related to impervious surface area (ISA) and cropland area (CLA) when ISA (CLA) was less than 160 (3000) km2. However, the relationship was positive when ISA (CLA) was larger than 160 (3000) km2, which provided a reasonable explanation for the observed spatial patterns of water quality. Distinct increasing temporal trends for two kinds of pollution indexes were also found. The annual ISA was significantly related to the rapid degradation of water quality from 1998 to 2008, with correlation coefficient (r) values of 0.816 (p = 0.002) and 0.711 (p = 0.014) for the organic pollution index (OPI) and IPI, respectively. However, annual rainfall was negatively correlated with the two indexes with r values of 0.785 (p = 0.002) and 0.448 (p = 0.093) for OPI and IPI, respectively. Our study highlights that decision makers should be more aware of recent increases in the pollution of the Xiangjiang River, especially at downriver sites and during low‐flow periods. Copyright © 2013 John Wiley & Sons, Ltd.

Spatial Distribution and Seasonal Variations of Heavy Metals in Surface Water of the Taizi River

The contamination levels and distribution of heavy metals in surface water of the Taizi River were investigated. Samples of water were collected from upstream to downstream locations, and also from the main tributaries of the Taizi River. All samples were analyzed for arsenic, mercury, and chromium (Ⅵ). Results indicated that the metal levels of upriver sites were lower than downriver sites. The total concentrations of Cr6+ and As in dry season was higher than that in flood season. While heavy metal Hg represents the highest concentration in flood season. Compared with the permissible levels of surface water environmental quality standards (GB3838-2002), the levels of Cr6+ and As attained the first and second level, but Hg attained the fifth level. This indicated that most sites of the Taizi River have a high risk of Hg pollution in flood period.

Biomass and Growth of Waterhyacinth in a Tidal Blackwater River, South Carolina

Abstract Eichhornia crassipes (Waterhyacinth) occurs in isolated populations along the Waccamaw River in northeast South Carolina. Although actively managed with herbicides, plant biomass and growth in this coastal, blackwater river have not been measured. We located three persistent populations in protected backwaters during spring 2009 and used sequential harvests to measure biomass accumulation and allocation. Relative growth over one month in existing populations and in two downriver sites was measured by placing plants in floating cages. A separate experiment was conducted to determine salinity tolerance. Mean total biomass in the persistent populations was relatively low but increased from 202.9 g/m2 in spring to 380.1 g/m2 in fall, with leaves as the largest biomass component (72%). Absolute growth and leaf nutrient content for nitrogen, phosphorus, and potassium were highest for caged plants placed in a downriver site influenced by the Pee Dee River, a redwater system. Our results suggest that Wat...

Downriver Passage of Juvenile Blueback Herring near an Ultrasonic Field in the Mohawk River

At the Crescent Hydroelectric Project (Crescent), New York, the Mohawk River is impounded by two dams separated by an island, which creates two channels; the turbine channel conveys water to the hydroelectric turbines, and the main channel conveys water around the turbines. Our objective was to determine whether ultrasound could be used to divert juvenile blueback herring Alosa aestivalis from entering the turbine channel during their fall downriver migration. Fixed-location hydroacoustics (420 kHz) was used to continuously monitor fish passage in the main channel upriver and downriver of the 122–128-kHz ultrasonic projectors. A relative index of abundance was derived from echo integration of acoustic backscatter collected from three horizontally aimed single-beam transducers sampling across each channel, while the proportion and speed of fish moving downriver were estimated from upward-facing split-beam transducers. The presence of juvenile blueback herring was verified by castnetting. Fish migrated downriver episodically for periods of hours rather than days. The mean daily number of downstream migrants (Nd) in the main channel was estimated to be 4.2 times higher at the upriver site than at the downriver site. The estimate for Nd at the main-channel downriver site (NDM) was compared with the expected Nd (NEDM) based on the proportion of total river flow moving through the main-channel downriver site as measured by acoustic Doppler current profilers. The mean daily difference between NDM and NEDM was significantly different from zero. The estimated proportion of fish that passed the main-channel downriver site (31.3%) was almost three times greater than the proportion expected (11.5%). If it is valid to assume that water flow directly influences entrainment and impingement, then the significantly higher-than-expected number of blueback herring that migrated downriver in the main channel could be an indication that ultrasound at Crescent was partially effective in diverting fish. Received July 17, 2011; accepted February 2, 2012

River dynamics shape clonal diversity and genetic structure of an Amazonian understorey herb

Summary 1. Clonal herbs are an important feature of the understorey of Amazonian forests. However, little is known about the environmental drivers determining the population genetics of these herbs and about the spatial scale of gene flow. 2. With amplified fragment length polymorphism markers, we analysed the clonal diversity and genetic structure of a clonal Amazonian herb (Heliconia metallica) in south‐eastern Peru at two spatial scales. First, we sampled leaves in 24 patches differing in flooding frequency and canopy openness in 2 km2 of a floodplain forest, and second in 21 riverine populations from the Andean foothills to the Amazon lowlands along a 550‐km stretch of river. 3. At the small spatial scale in the floodplain forest, the clonal diversity of H. metallica was higher at flooded than at non‐flooded sites, but clonal diversity did not increase with canopy openness. 4. At the large spatial scale, clonal diversity was very low in riverine populations at up‐ and downriver sites, suggesting that seedling recruitment was higher at mid‐altitudes where the flooding intensity is intermediate. Genetic diversity of riverine populations monotonously increased downriver, indicating unidirectional gene flow mediated by hydrochory. 5. Genetic differentiation among riverine populations was very low (FST = 0.06) and followed an isolation‐by‐distance pattern, indicating a stepping‐stone type of gene flow by seeds. Despite the much smaller spatial scale, genetic differentiation among patches in the floodplain forest was higher (FST = 0.16), due to spatially restricted gene flow in the forest understorey. 6. Synthesis. The genetic structure of H. metallica is the result of seedling recruitment being largely limited to flooded sites and of hydrochoric seed dispersal between populations growing on riverbanks. We conclude that river dynamics are the major determinant of the genetic structure of Amazonian plants and that largely undisturbed river systems, such as the Amazon, provide a crucial vector for gene flow, even at large spatial scales.

Anchoring Submersible Ultrasonic Receivers in River Channels with Stable Substrate

AbstractWe developed an anchoring system for submersible ultrasonic receivers (SURs) that we placed on the bottom of the riverine reaches of three main‐stem reservoirs in the upper Tennessee River. Each anchor consisted of a steel tube (8.9 × 35.6 cm) welded vertically to a round plate of steel (5.1 × 40.6 cm). All seven SURs and their 57‐kg anchors were successfully deployed and retrieved three times over 547 d by a dive team employing surface air‐breathing equipment and a davit‐equipped boat. All of the anchors and their SURs remained stationary over two consecutive winters on the hard‐bottom, thalweg sites where they were deployed. The SUR and its anchor at the most downriver site experienced flows that exceeded 2,100 m3/s and mean water column velocities of about 0.9 m/s.

Performance of Disease-Tolerant Strains of Eastern Oyster (Crassostrea virginica) in the Patuxent River, Maryland, 2003 to 2007

Three strains of specific pathogen-free (SPF) oysters (Crassostrea virginica) were set on shell and deployed on 3 oyster bars along a salinity gradient in the Patuxent River, MD, to determine whether oyster strains selected or bred for enhanced disease tolerance would outperform a generic control strain in survival and/or growth while under pressure from Perkinsus marinus (the agent of dermo disease). A local Standard oyster strain was used as a reference control, and selected oysters belonged to disease-tolerant DEBY and CROSBreed strains. Initial deployment was in September 2003, during a period of unusually low salinity. Mean river salinities during the first 2 y were 8.5‰ and 9.0‰, respectively, but these rose to historically typical levels of 11.5‰ and 11.3‰ during the third and fourth years. Although measuring possible differential effects of dermo disease pressure on the 3 strains was our primary goal, disease pressures were only slight or moderate during all but the final year of the study. Oysters grew well during most of the study, except at the upriver station, where salinities were less than 5‰ during much of the first year. The DEBY strain showed the best overall growth and survival. Standard and CROSBreed strains were similar in both growth and survival, and did not perform as well as DEBYs. Although P. marinus infection prevalences approached 90–100% among oysters at the 2 downriver sites, even during the first year, and were generally high in late summer each year, oyster body burdens of pathogen cells increased slowly throughout the study, with means reaching lethal levels of 106 parasite cells/g tissue only among Standards and DEBYs, and only during the fourth year. P. marinus infections among sympatric feral oysters (the probable source of disseminated cells infecting SPF oysters), were low during all years at the upriver site, but increased each year with increasing salinities at the downriver sites. As measured by better growth and survival, DEBYs performed best among the 3 strains tested. Whether DEBYs have a use in oyster restoration efforts is controversial, but their value in aquaculture applications appears evident.

Application of chemometric methods to analyze the distribution and chemical fraction patterns of metals in sediment from a metropolitan river

Rivers in metropolitan areas are often highly polluted with materials that pose a threat to a large number of residents. Human influences lead to contaminants in metropolitan rivers having more complex sources than those in rural rivers. This complexity results in contamination that is unstable and rapidly changing. Here, the contents and chemical fractionation patterns of eleven toxic elements (As, Cd, Co, Cu, Cr, Mn, Ni, Pb, Zn, Y, and Hg) were evaluated in 13 samples collected from along the Beiyunhe River in Beijing, China. The results revealed that the metal contents were unevenly distributed along the river, with higher levels being observed in the downriver sites and the rendezvous sites. Additionally, more than 80% of the metals were found to be in the residual phase. The organic and sulfide phases were the most important extractable phases of most metals, with Ni, Co, Cu, and Cr primarily being associated with these phases and As, Cd, and Zn having a strong association with the iron/manganese oxide and hydroxide phases. Additionally, Mn was associated with the exchangeable and carbonate phases, with the lowest concentrations being observed in the organic and sulfide phases. Conversely, the metal exchangeable and carbonate phases were uniformly distributed throughout the river. Analysis of the metal sources revealed that particles input from the atmosphere comprised a considerable amount of the metals in the Beiyunhe River. However, these metals likely do not enter the sediment via atmospheric deposition directly, but rather through rainwater runoff into the river. The methods used in the present study will be useful in other studies that require analysis of complex data.

Longitudinal changes in zooplankton distribution below a reservoir outfall with reference to river planktivory

The fate and interactions with river organisms of zooplankton as they drift downriver from a reservoir on a fourth-order mountain stream (Hiji River, Japan) were investigated. Monthly samples were collected at the reservoir and six river sites, simultaneously, from May 2005 to May 2006. Aquatic macroinvertebrates and fish were colleted, and their stomach contents were analyzed in April and May, 2006, respectively. Drift from the reservoir was the primary source for the river plankton community; the abundance of zooplankton, particularly those of cladocerans and large rotifer, rapidly decreased within several kilometers of the dam. Analysis of the contents of fish stomachs showed that drifting zooplankton was the main food for fish, with strong food selectivity for cladocerans and large rotifers. However, fish and insect planktivores showed longitudinally different stomach contents, with progressively fewer zooplankton found in the stomachs at the downriver sites. The results suggest that the outflow of zooplankton from the reservoir is an important food source for the downstream predators, especially fish, but the drift of zooplankton and consequent food availability for the predators at lower sites are strongly limited by concentrated fish predation just below the reservoir dam.

Spatial variation in population dynamics in a colonial ascidian (Botryllus schlosseri)

Recent interest in the dynamics of marine invertebrate populations has focused largely on taxa with an open population structure. However, in many colonial taxa with limited larval dispersal, settlers may be locally derived. Consequently, dynamics may vary among sites that are separated by relatively short distances. This study explored spatial variation in temporal dynamics of colonial ascidians (Botryllus schlosseri Pallas) inhabiting five sites distributed along a ≈ 17-km temperature and phytoplankton gradient in the Damariscotta River estuary, Maine, USA. Settlement and population densities and sexual reproductive status were assayed throughout the summer seasons of 1996 and 1997. Sexual reproduction and larval settlement commenced earlier in the summer in up-river populations, which subsequently underwent a seasonal population explosion that was much smalier in down-river populations. Two peaks in settlement density up-river (in early July and early September) suggest that colonies there may have completed two sexual generations, in contrast to a single generation at down-river sites. Similar spatial variation is expected among populations of other taxa with limited larval dispersal when they are distributed across environmental gradients.

An evaluation of a river health using the index of biological integrity along with relations to chemical and habitat conditions

We evaluated the health condition of a temperate river during June–November 1999 through applications of the index of biological integrity (IBI) using fish assemblages and qualitative habitat evaluation index (QHEI) as well as chemical analyses. Overall IBI values ranged from 13 to 37 and averaged 23 ( n=25, standard error=1.16), indicating a “poor” or “very poor” condition according to the criteria of modified Karr [Fisheries 6 (1981) 21]. The values of mean IBI declined at a rate of 0.22 km −1 ( R 2=0.91, p<0.05) along the longitudinal distance from the headwaters to the downstream sites. Reduced IBI values at downstream sites reflected low forest cover, high population density and high nutrient enrichments. Ecotoxicity tests using the river water also showed that toxic impacts were evident in the downriver sites. These factors resulted in decreases of riffle benthic species and insectivores and increases of tolerant species, anormalies and exotic species in the river. Spatial pattern in IBI agreed with QHEI values, which showed a linear relation ( R 2=0.998, p<0.001) with species richness. Field measurements of conductivity and pH, an indicators for variation of conservative ions, showed that the river water was diluted by 40% fold by summer monsoon rain and surface run-off from the watershed, resulting in a physical and chemical instability during the monsoon. For these reasons, average IBI values during the monsoon and postmonsoon decreased >20% compared to the premonsoon, indicating that IBI values were also affected by flow regime. Based on the overall physico-chemical data and IBI values, the river health is rapidly degrading due to the combined effect of chemical contaminations and habitat modifications.

BARSA RIVER MONITORING

Barsa River is one of the tributary streams of Olt River in Brasov region, which course is close to industrial areas with significant impact on environmental quality, Zarnesti, Rasnov and Brasov. Oxygen is a vitally important component in water for all the aquatic species. The oxidation of organic matter ({CH2O}), biooxidation of nitrogenous material (NH4+) and chemical or biochemical oxidation of reducing agents (Fe2+, SO32-) can deplete oxygen level. All these processes contribute to the deoxygenation of water; therefore pollution parameters of major importance in aquatic systems is the amount of dissolved oxygen (DO), biological oxygen demand in 5 days (BOD5) and chemical oxygen demand (COD), in different natural waters. This study presents Barsa River water quality focused on the DO, BOD5 and COD values as pollution parameters. Three sampling sites were monitored, up-stream Zarnesti, downriver Zarnesti and up-stream Olt River confluence. The results are presented as annual means, covering 1996-2000 period. The monitoring up-stream Zarnesti site showed that the parameters values corresponds to the natural value, as no pollution sources are noticed in this area. Between up-stream Zarnesti and down-river Zarnesti sites there are two important industrial pollution sources: S.C. Celohart S.A. Zarnesti and S.C. Tohan S.A. Zarnesti. DO, BOD5 and COD parameters show variable values, sometimes even worst than the accepted values for the III water category. More important industrial plants are in the up-stream Olt River confluence area, S.C. Romacril S.A. Rasnov and Brasov Water Treatment Plant, that can be pollution sources. However, the monitored parameters show better water quality compared with down-river Zarnesti, due to the natural oxygenation process, along the river stream.

Precipitation and River Water Chemistry of the Piracicaba River Basin, Southeast Brazil

Annual precipitation and river water volumes and chemistry were measured from 1995 to 1998 in a mesoscale agricultural area of southeast Brazil. Precipitation was mildly acidic and solute concentrations were higher in the west than in the east of the basin. Combustion products from biomass burning, automobile exhaust, and industry typically accumulate in the atmosphere from March until October and are responsible for seasonal differences observed in precipitation chemistry. In river waters, the volume-weighted mean (VWM) concentrations of major solutes at 10 sites across the basin were generally lower at upriver than at downriver sampling sites for most solutes. Mass balances for major solutes indicate that, as a regional entity, the Piracicaba River basin was a net sink of H+, PO4(3-), and NH4+, and a net source of other solutes. The main stem of the Piracicaba River had a general increase in solute concentrations from upriver to downriver sampling sites. In contrast, NO3- and NH4+ concentrations increased in the mid-reach sampling sites and decreased due to immobilization or utilization in the mid-reach reservoir, and there was denitrification immediately downriver of this reservoir. Compared with tributaries of the Chesapeake Bay estuary, the Piracicaba River is affected more by point-source inputs of raw sewage and industrial wastes than nonpoint agricultural runoff high in N and P. Inputs of N and C are responsible for a degradation of water quality at downriver sampling sites of the Piracicaba River drainage, and water quality could be considerably improved by augmenting sewage treatment.

Influence of salinity and temperature on the growth and production of a freshwater mayfly in the Lower Mobile River, Alabama

Secondary production of the burrowing mayfly, Hexagenia limbata, was quantified from four sites differing in seasonal salinity within the Lower Mobile River, Alabama, from October 1995 to September 1996. This population was univoltine, with emergence occurring from late May through early August. Comparisons with other populations of this species showed latitudinal trends suggesting that summer temperatures may exceed an upper thermal threshold for growth. Longitudinal differences in riverine salinity (i.e., upriver sites, 0‰; downriver sites, 5.5‰ maximum salinity) explained most of the differences among sites, both for average density (upriver sites, 75.6 mayflies m−2; downriver sites, 2.54 mayflies m−2) and annual production (upriver, 1,669 mg m−2 yr−1; downriver, 46.6 g m−2 yr−1). Laboratory bioassays indicated that H. limbata nymphs could survive elevated salinity (LC50 of 6.3‰ at 18°C; 2.4‰ at 28°C), although growth experiments showed similar growth at 0, 2, 4, and 8‰ salinity treatments. Results from field observations and laboratory experiments demonstrated that these mayflies are tolerant of increases in salinity and showed that individuals surviving the stress of elevated salinity can grow at similar rates as mayflies in freshwater.

Distribution of Nitrogen-Fixing Microorganisms along the Neuse River Estuary, North Carolina.

Nitrogen fixation genes (nifH) were amplified and sequenced from DNA extracted from surface water samples collected from six stations along the length of the Neuse River Estuary, North Carolina, in order to determine the distribution of nitrogen-fixing organisms in the transition from fresh- to saltwater. Nitrogenase genes were detected in all samples by a nested polymerase chain reaction method, and the amplification products from the upriver, midriver, and downriver stations were cloned, sequenced, and used for phylogenetic analysis. The composition of nifH clone libraries from upriver, midriver, and downriver stations (each composed of 14 randomly selected clones) were very diverse (samples from upriver and midriver stations were composed of 14 unique sequences, downriver station composed of 7 unique sequences) and differed among the stations. Some phylotypes were found at more than one station, but were usually found in the upriver and midriver stations or in the midriver and downriver stations, indicating that the phylotypes were probably transported along the river. Cyanobacterial nifH were not found at the most upriver site, but were a large fraction of sequences (50%) recovered from the downriver station, where nitrate concentration was an order of magnitude lower and salinity was higher. In contrast, g proteobacteria nifH sequences were much more common at the midriver and upriver sites (58% and 64%, respectively), compared to the downriver site (14%). Results indicate that substantially different nitrogen-fixing assemblages are present along the river, reflecting differential watershed hydrological inputs, sedimentation, and environmental selection pressures, along the salinity gradient.

Seasonal pulses of turbidity and their relations to eelgrass ( Zostera marina L.) survival in an estuary

The light environment of one Chesapeake Bay tributary where seagrasses have decreased in abundance was described using both continuous and discrete measures of irradiance and related to the growth and survival of transplanted eelgrass ( Zostera marina L.). After 8 months of continuous growth at an upriver site, a decline and eventual complete loss of eelgrass transplants began during a month long (May–June) period of increased turbidity ( K d>3.0). Transplant loss continued even after light conditions improved ( K d<2.0). At a downriver site where there has been some natural seagrass regrowth, the pulse of high turbidity was not as evident and transplants survived. Other than this spring period of high turbidity at the upriver site, the light environments of the two areas were similar with minimum turbidity in January and maximum in the spring and summer. Annual median daily attenuation coefficients ( K d) at the upriver and downriver sites were 1.77 and 1.96, respectively, and were not significantly different ( P=0.49). Total downwelling quantum flux at transplant depths of 0.8 m below mean sea level were 2618 and 2556 mol·m −2·yr −1, or approximately 24.9 and 24.3% of annual solar PAR. The high spring turbidity pulse corresponded to an increase in non-chlorophyll particulate matter. Chlorophyll specific attenuation ( K c) accounted for 6.7–9.0% of K d in June. Differences in attenuation were greatest in the 400–500 nm spectral region. Therefore, measures of total PAR attenuation can overestimate the usable irradiance available to the macrophytes. Scalar quantum fluxes during the period of elevated turbidity were 2.7 and 13.4 mols·m −2·day −1 at the upriver and downriver sites. The duration and intensity of total PAR measured upriver during this period were insufficient to support eelgrass growth and survival, and below literature estimates for eelgrass community light compensation at in situ temperatures (20–25°C). Therefore late spring, month-long pulses in turbidity, such as measured here can account for the loss of transplanted vegetation and, potentially, explain lack of successful recruitment into formerly vegetated upriver sites.

Growth and mortality of cultured bay scallops in the Damariscotta River, Maine (USA)

Growth and mortality of hatchery-reared bay scallops, Argopecten irradians, were determined over an 18 month period (July 1994-December 1995) at two locations in the Damariscotta River, Maine, north of its natural distribution. Growth occurred primarily between May and October when mean water temperature exceeded 10 °C. Final mean shell height (56.3 mm) was significantly greater (p ≤ 0.0005) at the upriver site than at the downriver site in association with a higher mean water temperature. Cumulative mortality (43-49%) over this period occurred primarily over the winter months, in conjunction with mean water temperatures below 5 °C. Although it is possible to rear bay scallops to commercial size in Maine, the short growing season, high winter mortality, and costs associated with over-wintering and fouling will likely deter commercial culture.