Water Column Salinity Research Articles

Distribución espacio-temporal de la meiofauna béntica en cuatro playas del Litoral Norte de la Habana

The temporal-spatial distribution of benthic meiofauna was evaluated in four beaches at the north coast of Havana, Cuba, from March 2003 to February 2004. We studied two urban beaches (Santa Fe and La Concha) and two tourist beaches (Mar Azul and Canasi). Monthly meiofauna samplings were taken by scuba-diving using with a syringe (inner diameter 2.5 cm), and physico-chemical parameters (grain size, interstitial salinity and water column salinity were recorded with standard equipment). Statistical analysis (MDS and ANOSIM) were performed. Depth and biotope kind were the same in the four beaches. Highest densities were obtained in Santa Fe (7,133.48 ind/10 cm2) while the lowest mean densities were found in Canasí (892.12 ind/10 cm2). We recorded 13 taxa; the dominant organisms in Santa Fe and Mar Azul were free-living marine nematodes. Copepods were the dominant organisms in La Concha and Canasí.

Climate Drivers of Spartina alterniflora Saltmarsh Production in Georgia, USA

Tidal wetlands are threatened by global changes related not only to sea level rise but also to altered weather patterns. To predict consequences of these changes on coastal communities, it is necessary to understand how temporally varying abiotic conditions drive wetland production. In 2000–2011, we conducted annual surveys of Spartina alterniflora biomass in tidal marshes at nine sites in and around the Altamaha river estuary on the coast of Georgia, USA. End of the year live biomass was assessed in the creekbank and midmarsh zones to estimate annual net primary production (ANPP). River discharge was the most important driver of S. alterniflora ANPP, especially in creekbank vegetation. Increased river discharge reduces water column salinity, and this was most likely the proximate driver of increased production. In the midmarsh zone, the patterns were less distinct, although river discharge was again the best predictor, but maximum temperature had similar predictive ability. In contrast to results from terrestrial grasslands, we found no consistent evidence for a sharply delimited critical period for any climate driver in the tidal marsh, which indicates that plant growth was responsive to abiotic drivers at any time during the growing season. Results were broadly consistent across multiple sites within a geographic region. Our results differ from previous analyses of production in S. alterniflora marshes, which either identified oceanic drivers of S. alterniflora production or were unable to identify any drivers, likely because the low-latitude sites we studied were hotter and more affected by river discharge than those in previous studies.

Seasonality and environmental drivers of biological productivity on the western Hokkaido coast, Ishikari Bay, Japan

We report on a 16-month time series of the primary productivity, chlorophyll biomass and environmental variables in Ishikari Bay, the western Hokkaido coast, from September 2006 to December 2007. During productive summer and autumn seasons, the average phytoplankton biomass (as chlorophyll a) and productivity were 4.7 ± 0.4 μg Chl a L−1 and 229.2 ± 113.4 μg C L−1 d−1, and 5.9 ± 6.0 μg Chl a L−1 and 138.9 ± 202.6 μg C L−1 d−1, respectively. The water column salinity profiles suggested that nutrient upwelling from bottom waters, especially in summer and autumn, when autotrophic production was at a maximum, was the major source of nutrients fueling the productivity in near-surface waters. Size-fractionation measurements showed that the micro-size fraction (>10 μm) accounted for > 80% of the total phytoplankton biomass and productivity. The strong correlation of POC with Chl a (r = 0.875; P < 0.001; n = 16) suggests that living phytoplankton in POC may outweigh detrital and bacterial biomass. Stoichiometrical analysis of inorganic nutrients revealed a seasonal low (13) and high (25) DIN-to-PO4 ratio and a seasonal low (30) and high (37) Si-to-PO4 ratio, suggesting that phytoplankton growth was possibly limited by nitrate and phosphate and not by silicic acid. We conclude that bottom-advection of nutrients resulting from wind-driven mixing may have been a significant source of nutrient input in this oligotrophic coastal system, and zooplankton grazing or other losses might be a crucial factor in controlling the phytoplankton biomass and production, particularly in spring. Also, it is probable that phytoplankton growth was not prevented by insufficient light in Ishikari Bay considering the values of both mean light intensity in the mixed layer (Im) and the mixing and euphotic depth ratio (Zmix:Zeu).

Apparent loss of Vibrio vulnificus from North Carolina oysters coincides with a drought-induced increase in salinity.

Despite years of successful isolation of Vibrio vulnificus from estuarine waters, beginning in 2007, it was extremely difficult to culture V. vulnificus from either North Carolina estuarine water or oyster samples. After employing culture-based methods as well as PCR and quantitative PCR for the detection of V. vulnificus, always with negative results, we concluded that this pathogen had become nearly undetectable in the North Carolina estuarine ecosystem. We ensured that the techniques were sound by seeding North Carolina oysters with V. vulnificus and performing the same tests as those previously conducted on unadulterated oysters. V. vulnificus was readily detected in the seeded oysters using both classes of methods. Furthermore, oysters were obtained from the Gulf of Mexico, and V. vulnificus was easily isolated, confirming that the methodology was sound but that the oysters and waters of North Carolina were lacking the V. vulnificus population studied for decades. Strikingly, the apparent loss of detectable V. vulnificus coincided with the most severe drought in the history of North Carolina. The drought continued until the end of 2009, with an elevated water column salinity being observed throughout this period and with V. vulnificus being nearly nonexistent. When salinities returned to normal after the drought abated in 2010, we were again able to routinely isolate V. vulnificus from the water column, although we were still unable to culture it from oysters. We suggest that the oysters were colonized with a more salt-tolerant bacterium during the drought, which displaced V. vulnificus and may be preventing recolonization.

Natural trace elemental markers for adult red rock lobsters Jasus edwardsii vary among replicate distinct water masses

In order to determine the potential utility of trace elemental natural tags to resolve fine-scale stock structure and residency patterns in adult crustaceans, we investigated whether concentrations of trace elements in exoskeleton could be used to resolve capture sites of red rock lobsters Jasus edwardsii. Using inductively coupled plasma mass spectrometry, we measured ratios of Li, Mg, Mn, Ba and Sr to Ca in samples of exoskeleton from 4 pairs of inner fjord and outer coast sites in Fiordland on New Zealand's southwest coast. Water column salinity structure was characterised at these 8 sites from a 12 yr series of conductivity, temperature and depth casts. Information on capture site, sex, carapace length and moult stage was used in an information theoretic framework to assess the degree to which capture site could be statistically discriminated using trace element signatures of exoskeleton. Using a permutational multivariate analysis of variance, we found significant differences in trace elemental signatures of exoskeleton at the scale of 10 to 20 km for 2 of the 4 fjords. In these same 2 fjords, we found persistent differences in water column salinity structure between sites. A multivariate permutational assignment test (canonical analysis of principal coordinates) successfully reclassified individuals from all 4 pairs of sites better than chance. These results indicate that trace elements can effectively be used to discriminate capture sites of adult rock lobsters between regions with different water mass characteristics. Within these limits, trace elements of exoskeleton may prove effective as natural tags to provide valuable information on fine-scale stock structure and residency patterns for rock lobsters.

Diatom and foraminifera relationships to water quality in The Coorong, South Australia, and the development of a diatom-based salinity transfer function

The Ramsar-listed Coorong lagoon lies at the terminus of the Murray-Darling River system in South Australia. Diatom and foraminifera relationships with water quality were characterised in order to develop diatom- and foraminifera-based models with the potential to infer water column salinity. Seventy-four samples were collected during 2007, a year of continuing drought in the catchment, and of no discharges at the Murray Mouth. The sample sites had a salinity gradient of 1.8–190 g l−1 total dissolved solids. The diatom data set comprised 215 taxa, while there were only eight taxa in the foraminiferal data set. Canonical correspondence analysis of diatom species-environment relationships showed that salinity explained the largest proportion of diatom variance. Hence, a diatom-based salinity transfer function with reasonable predictive power (measured vs. diatom-inferred salinity r jack 2 = 0.82; Root Mean Squared Error of Prediction = 16 g l−1) was developed. Application of the transfer function to fossil diatom assemblages from The Coorong suggested that pre-European salinity values were generally >50 g l−1 and that salinity declined following settlement. These results, however, contradict the recent history of The Coorong where there have been substantial lagoon-wide salinity increases. The pre-impact diatom flora has no analogue in the modern data set, highlighting the degree of departure from past conditions. CCA of the foraminiferal data set identified salinity and total nitrogen as the variables with the greatest explanatory power. However, accurate predictive models could not be developed using either variable due to low foraminiferal abundance and species richness. These factors may have been a consequence of diminished foraminiferal recruitment rates over successive years, an artefact of reduced marine water input to The Coorong. Future attempts to generate predictive models from this region would benefit from the inclusion of data from distant locations, since suitable analogue sites do not exist in close proximity. The study has generated useful insights to the apparently broad salinity tolerances for several cosmopolitan diatom and foraminifera species, and has identified a number of diatom and foraminifera taxa that may prove useful in the qualitative interpretation of down-core trends in The Coorong and the lower Murray River region.

Zooplankton trends in Massachusetts Bay, USA: 1998-2008

Zooplankton community analyses in Massachusetts Bay (Gulf of Maine, USA) from 1998 to 2008 with 102 μm mesh nets revealed seasonal and interannual variability in zooplankton abundance, with maxima in summer and minima in winter. Zooplankton was dominated by copepod nauplii, copepodites and adults, mostly of the small cyclopoid copepod Oithona similis. Time-series analyses revealed a multi-year decline in total zooplankton abundance from 2000 to 2008, especially due to declines in Oithona similis and Pseudocalanus spp. This decline was not exhibited by barnacle nauplii and Calanus finmarchicus. There were no significant relationships between surface or mean water column temperature and salinity for any zooplankton group except for significantly higher abundances of C. finmarchicus at higher surface salinity. There was a significant positive correlation between total zooplankton abundance and integrated chlorophyll. The decline in zooplankton abundance since 2000 was not obviously related to diversion of sewage effluent from the Boston Harbor outfall to Massachusetts Bay, or to natural hydrographic variability. The zooplankton decline in Massachusetts Bay highlights the importance of multi-year sampling to characterize interannual variability. Comparisons of our results with those of other programs in the Gulf of Maine are complicated by differences in sampling net meshes, but our results shared some similarities with other studies during years when these studies overlapped with ours. In particular, small copepods declined in abundance beginning around 2000-2001, Calanus finmarchicus was abundant and salinities were high during 2002 through 2004, and zooplankton and chlorophyll levels declined from 2000 through 2004.

The Potential for Using Little Diomede Island as a Platform for Observing Environmental Conditions in Bering Strait

The Pacific waters that enter the Arctic via the Bering Strait exert a major influence on the Arctic Ocean’s stratification, ice cover, and ecosystem. We demonstrate the potential of a shore-based laboratory to monitor the water masses that flow predominantly northward past Little Diomede Island in the center of the Bering Strait into the Arctic Ocean. We determined near-surface water column salinity, inorganic nutrient concentrations, natural fluorescence associated with chlorophyll, and the oxygen isotope composition of seawater, both in summer during the open-water period and in late winter under ice-covered conditions, by pumping ashore water from shallow depths near the island. Additional surveys were undertaken within 5 km of the island to assess the influence of local sources of nutrients. Water mass variability was much greater during the open-water period than under ice-covered conditions, presumably because the relatively immobile ice cover attenuates wind forcing and the decrease in run-off reduces cross-shelf gradients. The mean oxygen isotope composition of the summer (?18O = -1.11‰) and late winter (?18O = -0.98) collections, however, was close to that which has been established for Bering Sea waters in the Pacific-dominated upper halocline of the Arctic Ocean (-1.1‰) particularly considering the higher seasonal flow of runoff in the summer. A comparison with data from shipboard sampling at various locations across the Bering Strait indicates that the oxygen isotope composition of near-surface water sampled at Diomede varies in response to wind-forcing. If the least saline (&lt; 30.5) water near the Alaska coast is excluded, the ?18O values of Diomede and shipboard samples cannot be distinguished statistically. This similarity suggests that the water sampled from the island also reasonably represents the ?18O value of Bering Sea waters that contribute to the upper halocline of the Arctic Ocean. Effects of benthic recycling, human activity, and seabird nesting on nutrient concentrations appeared to be concentrated within ~200 m of the island. Our results are discussed in the practical context of availability of electricity, interested local residents, and a geotechnical study indicating that it is feasible to construct and operate a more permanent undersea water intake system to improve environmental observation capabilities in the Bering Strait region.

Microfossils and molecular records in oil shales of the Songliao Basin and implications for paleo-depositional environment

Several oil shale beds, over 10 m thick, occur at the base of the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) in the Songliao Basin. They act both as excellent source rocks for conventional oil and as potential oil deposit for shale oil production. Here we combine micropaleontology with organic geochemistry to investigate the paleo-depositional environment and organic source characteristics of the oil shales and black shales. Our results indicate that algal remains are dominant microfossils in K2qn1 oil shales, and their relatively high abundance suggests a major algal thriving event during the oil shale deposition. The presence of fresh water and brackish water species, Sentusidinium, Vesperopsis and Nyktericysta, and marine or brackish water deltaic and lagoonal species such as Kiokansium and Dinogymniopsis demonstrate that this paleo-continental lake was influenced by marine transgressions at the time of K2qn1 oil shale formation. The extremely low pristine/phytane ratios, relatively high abundance of gammacerane and 4-methyl steranes, and low δ13C values of C14-C37 n-alkanes in the oil shale organic extracts indicate the deposition of oil shales in anoxic and highly stratified water columns and the significant contribution of lacustrine algae to sedimentary organic matter. High molecular-weight paraffinic hydrocarbons with unusually high abundance of nC43, nC45, and nC47 may be related to special algal species associated with marine transgression events. The giant water body of Songliao paleo-lake and the change in the organic and chemical environment (such as nutrition source and water column salinity) associated with seawater transgression into the lake are among the most important reasons for oil shales in the Songliao Basin being different from mudstone and oil shale in other rifted basins.

Modelling physico-chemical factors affecting occurrences of a non-indigenous planktonic copepod in northeast Pacific estuaries

Estuarine ecosystems along the Pacific coast of North America are vulnerable to invasions by non-indigenous planktonic copepods, with documented invasions by at least nine species introduced via ship’s ballast. One of these, the calanoid copepod Pseudodiaptomus inopinus, now occurs in a relatively wide geographical area in coastal estuaries of Washington and Oregon States. Although it appears to be well established in the region, plankton surveys conducted in 1992, 1996, 2000, and 2004 in estuaries from southern Vancouver Island in British Columbia, Canada, to northern California, United States indicate that it has not expanded its range. This static distribution suggests that P. inopinus has reached a distributional limit, and it may thus be a good organism for applying models for predicting planktonic invasions, by characterizing estuaries with and without populations of the copepod. In this study, we applied both parametric, linear (discriminant function analysis, logistic regression) and nonparametric, non-linear (classification trees) techniques to develop models for occurrence of P. inopinus, to identify parameters that may lead to successful invasions and to identify specific estuaries or regions that might be at risk of invasion by this species. Both model types had similar results, identifying relatively simple salinity- and stratification-based models as good predictors of P. inopinus. While different models selected slightly different sets of variables and thresholds, all models identified relatively low salinity and stratification of water column temperature and salinity as important predictors of P. inopinus presence. The models also identified several “false positives” that mainly occurred in more inland waters of Puget Sound—estuaries that did not have P. inopinus, but had the conditions that support it, and which may be at risk for future invasions by this species.

Supercooled water in an Arctic polynya: observations and modeling

AbstractIn situ field measurements of an active polynya in Storfjorden, Svalbard, during April 2006 are presented. A surface heat flux, estimated to be 400 W m−2, produced frazil ice that was advected away from the fast ice edge during the end of a polynya event driven by cold winds from the northeast. Conductivity, temperature and depth casts from the fast ice edge of the polynya were calibrated by accompanying water samples, and reveal a supercooling event that lasted for 3 days in a 5 m deep water column. Surface salinity reached 35.9 psu from brine release during ice growth. The maximum supercooling measured was 0.037 ± 0.005°C below the in situ freezing point near the surface and 0.016 ± 0.005°C at the bottom; the mean supercooling gradient was 0.020 ± 0.005°C between the surface and the bottom. These measurements are consistent with results from a one-dimensional frazil ice model, confirming that such supercooling levels can be expected. Frazil ice concentrations in the water were modeled to be lower than 0.02 g L−1, due to advection in the surface layer. Seven frazil/grease ice samples taken from a place where advection was blocked along the fast ice edge showed a mean salinity of 26.2 psu, indicating 25% frazil ice and 75% sea water in the grease ice. The water-column salinity decreased during the measurement period due to less saline water replacing newly formed brine-enriched shelf water flowing down to deeper parts of Storfjorden. The supercooling ceased when the wind direction turned to the east, with higher air temperatures and warmer and less saline water being pushed into Storfjorden by the northward Ekman transport. These are the first in situ observations from an active Arctic polynya with concurrent sampling of hydrography and frazil ice, and the supercooling is the maximum observed in recent years with modern and accurate instrumentation.

Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 363:205-215 (2008) - DOI: https://doi.org/10.3354/meps07455 Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge Ryan K. Dale, Douglas C. Miller* University of Delaware, College of Marine and Earth Studies, 700 Pilottown Rd Lewes, Delaware 19958, USA *Corresponding author. Email: dmiller@udel.edu ABSTRACT: Groundwater discharge is a common phenomenon along sandy coasts. At Cape Henlopen, Delaware, USA, it creates low porewater salinity in spatially heterogeneous patterns over 1 to 10 m horizontally. In the present study, porewater salinity and macrofaunal communities were sampled along intertidal transects in summer and spring. We consistently observed high spatial variability in porewater salinity at 10 cm sediment depth, with changes up to 15.8 m–1. Community composition and species abundance differed between the seasons, but, in each season, communities associated with low porewater salinity (<15) differed from those associated with higher porewater salinity (15 to 24 or >24). In general, species from a diverse pool of sand flat species gradually became absent as salinity decreased. These local community changes correlated with porewater salinity could compromise the interpretation of various benthic indices that are based on water-column salinity, which, at this site, remains around 28. In most samples, the polychaete Marenzelleria viridis was numerically dominant (up to 10000 m–2) in burrows up to 50 cm deep. Both in the field and in cores constructed in the laboratory, we found that the number of worm burrows was highly correlated with hydraulic conductivity of the sediment. We propose that burrowing worms (especially M. viridis) at this groundwater discharge site act as hydraulic ecosystem engineers by means of their burrowing and tube building. In general, there appear to be strong physical–biological interactions among the distribution of organisms, their infaunal behavior, the hydrological properties of the sediment and the magnitude of groundwater discharge that may contribute to infaunal zonation on intertidal sand flats. KEY WORDS: Benthic communities · Groundwater · Salinity · Species distribution · Intertidal · Hydraulic conductivity Full text in pdf format PreviousNextCite this article as: Dale RK, Miller DC (2008) Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge. Mar Ecol Prog Ser 363:205-215. https://doi.org/10.3354/meps07455 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 363. Online publication date: July 15, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Assessing urban stormwater toxicity: methodology evolution from point observations to longitudinal profiling

The quality of aquatic habitat in a stormwater management facility located in Toronto, Ontario, was assessed by examining ecotoxicological responses of benthic invertebrates exposed to sediment and water from this system. Besides residential stormwater, the facility receives highway runoff contaminated with trace metals, polycyclic aromatic hydrocarbons (PAHs), and road salt. The combined flow passes through two extended detention ponds (in series) and a vegetated outlet channel. Toxicity of surficial sediment collected from 14 longitudinally arrayed locations was assessed based on 10 acute and chronic endpoints from laboratory tests with four benthic organisms. Greatest overall toxicity was observed in sediment from sites in the upstream pond, where mortality to amphipods and mayflies reached up to 100%. Downstream pond sediment was less toxic on average than the upstream pond sediment, but not the outlet channel sediment where untreated stormwater discharges provided additional sources of contamination. Macroinvertebrate communities in sediment cores were depauperate and dominated by oligochaetes and chironomids, with minimum densities and diversity at the deeper central pond sites. While sediment toxicity was associated with high concentrations of trace metals and high-molecular weight PAHs, benthic community impoverishment appeared related to high water column salinity.

Active methane venting observed at giant pockmarks along the U.S. mid-Atlantic shelf break

Detailed near-bottom investigation of a series of giant, kilometer scale, elongate pockmarks along the edge of the mid-Atlantic continental shelf confirms that methane is actively venting at the site. Dissolved methane concentrations, which were measured with a commercially available methane sensor (METS) designed by Franatech GmbH mounted on an Autonomous Underwater Vehicle (AUV), are as high as 100 nM. These values are well above expected background levels (1–4 nM) for the open ocean. Sediment pore water geochemistry gives further evidence of methane advection through the seafloor. Isotopically light carbon in the dissolved methane samples indicates a primarily biogenic source. The spatial distribution of the near-bottom methane anomalies (concentrations above open ocean background), combined with water column salinity and temperature vertical profiles, indicate that methane-rich water is not present across the entire width of the pockmarks, but is laterally restricted to their edges. We suggest that venting is primarily along the top of the pockmark walls with some advection and dispersion due to local currents. The highest methane concentrations observed with the METS sensor occur at a small, circular pockmark at the southern end of the study area. This observation is compatible with a scenario where the larger, elongate pockmarks evolve through coalescing smaller pockmarks.

Impacts of urban wastewater discharge on seagrass meadows ( Zostera noltii)

The abiotic disturbance of urban wastewater discharge and its effects in the population structure, plant morphology, leaf nutrient content, epiphyte load and macroalgae abundance of Zostera noltii meadows were investigated in Ria Formosa coastal lagoon, southern Portugal using both univariate and multivariate analysis. Four sites were assessed, on a seasonal basis, along a gradient from a major Waste Water Treatment Works (WWTW) discharge to a main navigation channel. The wastewater discharge caused an evident environmental disturbance through the nutrient enrichment of the water and sediment, particularly of ammonium. Zostera noltii of the sites closest to the nutrient source showed higher leaf N content, clearly reflecting the nitrogen load. The anthropogenic nutrient enrichment resulted in higher biomass, and higher leaf and internode length, except for the meadow closest to the wastewater discharge (270 m). The high ammonium concentration (158–663 μM) in the water at this site resulted in the decrease of biomass, and both the leaf and internode length, suggesting a toxic effect on Z. noltii. The higher abundance of macroalgae and epiphytes found in the meadow closest to the nutrient source may also affect the species negatively. Shoot density was higher at the nutrient-undisturbed site. Two of the three abiotic processes revealed by Principal Component Analysis were clearly related to the WWTW discharge, a contrast between water column salinity and nutrient concentration and a sediment contrast between both porewater nutrients and temperature and redox potential. A multiple regression analysis showed that these abiotic processes had a significant effect on the biomass-density dynamics of meadows and on the overall size of Z. noltii plants, respectively. Results show that the wastewater discharge is an important source of environmental disturbance and nutrients availability in Ria Formosa lagoon affecting the population structure, morphology and N content of Z. noltii. This impact is spatially restricted to areas up to 600 m distant from the WWTW discharge, probably due to the high water renewal of the lagoon.

Understanding spatial and temporal dynamics of key environmental characteristics in a mesotidal Atlantic estuary (Douro, NW Portugal)

Ecosystem management and decision making process are dependent of a good knowledge of ecosystem functioning. Conceptual models allow knowledge organization through representation of relationships between variables and processes, facilitating management decisions. In this study, spatial and temporal characterization of key environmental variables, as well as relationships between them, was studied aiming the design of a conceptual model of the Douro estuary. This temperate mesotidal estuary is limited upstream by a hydroelectric power dam that controls freshwater inflow and prevents the propagation of the tide upstream, which, in turn, influences water circulation and biogeochemical dynamics of the system. During one year, from December 2002 to December 2003, water column data were collected monthly at 10 stations along the estuary, during ebb and flood tides. Spatial and temporal variability of water column salinity, temperature, nutrients, phytoplankton biomass, total particulate matter (TPM), phytoplankton primary production (PP), faecal coliform bacteria (FC) and community respiration (CR) were analysed. Salinity stratification was assessed by means of the Estuary Number (Ne) and variation of this index as well as other key characteristics with river flow was analysed. Freshwater discharge controlled salinity stratification and freshwater residence time. Ne indicated that the Douro was stratified for river flows <300 m 3 s −1, and freshwater residence time was >1 day for the same conditions. A decaying exponential relationship between PP and river flow was found, whereas nitrate and TPM increased logarithmically and linearly, respectively, with river flow. Regarding spatial distribution, nitrate and PP decreased downstream, showing that the river was a source of nutrients and phytoplankton, while the opposite trend was found for TPM, FC, ammonium and CR. The latter increase was probably due to untreated sewage discharge in the urbanized middle and lower estuarine stretches. Reduction of nitrate coming from the watershed and of bacterial contamination in the urban stretches of this highly modified water body, according to the European Water Framework Directive, emerges as the main water quality issues for this estuarine ecosystem.

Conceptual model of seagrass die-off in Florida Bay: Links to biogeochemical processes

Seagrasses are recognized as important plant communities in coastal estuaries and lagoons across both tropical and temperate climes; thus, large-scale seagrass die-off events worldwide are of general concern. In Florida Bay, at the southern terminus of the Florida peninsula, seagrass die-off events up to 4000 ha have been reported and smaller scale mortality events are noted annually. In the present study, we examined several hypothesized causative factors (high temperature, hypersalinity, sulfide toxicity) of seagrass ( Thalassia testudinum) mortality in Florida Bay. To test sulfide effects, in situ sulfide production was stimulated by applying a labile carbon source (glucose) to sulfate reducers in the sediment at five sites across the bay (northeastern, northcentral, and southwestern basins). During the one year study, high temperature (32–36 °C) and salinity (> 50 psu) were recorded in the bay associated with a regional drought. We also experienced major seagrass die-off events at two of our southwestern bay sites. These field conditions provided an excellent opportunity to closely examine cause–effect relationships among stressors and die-off events in the field, and verify results of our previous mesocosm experiments. Even though glucose amendments stimulated porewater sulfides in bay sediments (4–8 mmol L − 1 ), no significant differences in biomass, short shoot density or final growth rates were found between control and glucose plots. In addition, the highest growth rates and shoot densities were concomitant with maximum water column salinity (> 50 psu) and temperature (32–36 °C), when porewater sulfides were also in the millimolar range. Large-scale seagrass mortality events, encompassing ∼ 50% of the entire meadow at one site, occurred at southwestern bay sites when plants were down regulating (slower growth and shoot density), probably in response to shorter day length and lower temperature (30–34 to 23–26 °C) from October, 2004 to January, 2005. Sulfate reduction rates (SRR) were also 2-fold higher in the southwestern (214–488 nmol cm − 3 d − 1 ) versus northcentral and northeastern (97–240 nmol cm − 3 d − 1 ) bay sites, possibly limited by labile carbon, which we found to stimulate SRR 3-fold in northeastern and northcentral bay sites (461–708 nmol cm − 3 d − 1 ) and 4-fold at southwestern bay sites (1211–2036 nmol cm − 3 d − 1 ). Based on a synthesis of the field data reported herein, our mesocosm experiments to date, and contributions by others, we present a conceptual model of seagrass die-off in Florida Bay outlining a cascade of stressors, stimulated by P enrichment, which leads to high O 2 consumption in the system triggering a seagrass die-off event.

Hyperspectral Reflectance Response of Freshwater Macrophytes to Salinity in a Brackish Subtropical Marsh

Coastal freshwater wetlands are threatened by increased salinity due to relative sea level rise and reduced freshwater inputs. Remote radiometric measurement of freshwater marsh canopies to detect small shifts in water column salinity would be useful for assessing salinity encroachment. We measured leaf hyperspectral (300-1100 nm) reflectance of freshwater macrophytes (cattail, Typha latifolia and sea oxeye, Borrichia frutescens) in a field study in a subtropical brackish (2.5-4.5 parts per thousand salinity, per thousand) marsh to determine salinity effects on visible and near-infrared spectral band reflectance and to identify reflectance indices sensitive to small (1 per thousand) changes in wetland salinity. For sea oxeye, floating-position water band index [fWBI = R(900)/minimum(R(930) - R(980)), where R(lambda) = reflectance at band lambda], normalized difference vegetation index [NDVI = (R(774) - R(681))/(R(774) + R(681))], and a proposed wetland salinity reflectance ratio (WSRR = R(990)/R(933)) were sensitive to salinity with R2 of 40, 35, and 65%, respectively (p < 0.01). For cattail, NDVI and photochemical reflectance index [PRI = (R(531) - R(570))/(R(570) + R(531))] were sensitive to salinity with R2 of 29 and 33%, respectively (p <or= 0.01). Higher salinity significantly reduced mean reflectance of sea oxeye in 328- to 527-nm and 600- to 700-nm wavebands (p < 0.05), which corresponded to chlorophyll bands. Reflectance of cattail was not significantly affected by the highest salinity, although the spectral band most affected was 670 nm (p < 0.10), which is a chlorophyll a band. Our findings indicate that hyperspectral radiometry can detect the response of emergent freshwater plants to changes in wetland salinity, which would help with monitoring salinity effects on coastal wetlands.

Interactive effects of salinity and water depth on the growth of Melaleuca ericifolia Sm. (Swamp paperbark) seedlings

Melaleuca ericifolia Sm. (Swamp paperbark) is a common tree species in freshwater and brackish wetlands in southern and eastern Australia. The survival of this species in many wetlands is now threatened by increased salinity and inappropriate water regimes. We examined the response of 5-month-old M. ericifolia seedlings to three water depths (exposed, waterlogged and submerged) at three salinities (2, 49 and 60 dS m −1). Increasing water depth at the lowest salinity did not affect survival, but strongly inhibited seedling growth. Total biomass, leaf area and maximum root length were highest in exposed plants, intermediate in waterlogged plants and lowest in submerged plants. Although completely submerged plants survived for 10 weeks at the lowest salinity, they demonstrated negative growth rates and were unable to extend their shoots above the water surface. At the higher salinities, M. ericifolia seedlings were intolerant of waterlogging and submergence: all plants died after 9 weeks at 60 dS m −1. Soil salinities increased over time, and by Week 10, exceeded external water column salinities in both the exposed and waterlogged treatments. In exposed sediment, ∼90% of plants survived for 10 weeks at 60 dS m −1 even though soil salinities reached ∼76 dS m −1. No mortality occurred in the exposed plants at 49 dS m −1, and small but positive relative growth rates were recorded at Week 10. We conclude that at low salinities M. ericifolia seedlings are highly tolerant of sediment waterlogging, but are unlikely to tolerate prolonged submergence. However, at the higher salinities, M. ericifolia seedlings are intolerant of waterlogging and submergence and died rapidly after 5 weeks exposure to this combination of environmental stressors. This research demonstrates that salinity may restrict the range of water regimes tolerated by aquatic plants.

Spatial and temporal distributions of epiphytic diatoms growing on Thalassia testudinum Banks ex König: relationships to water quality

The spatial and temporal distributions of the epiphytic diatom flora on Thalassia testudinum was described within the Florida Bay estuary and at one Atlantic site east of the Florida Keys over a 1-year period. Species of the genus Mastogloia dominated the epiphytic diatom flora (82 out of 332 total species). Nonmetric Multidimensional Scaling (NMDS) and Analysis of Similarity (ANOSIM) revealed four distinct spatial assemblages and two temporal assemblages. Eastern and western Florida Bay assemblages were identified within the estuary. The eastern diatom assemblage was characterized by high relative abundances of Brachysira aponina and Nitzschia liebetruthii, while the western assemblage was characterized by the abundance of Reimerothrix floridensis, particularly during summer. Two diverse and distinct marine assemblages, one located in the Gulf of Mexico along the western edge of Florida Bay and the other behind the Florida reef tract in the Atlantic Ocean, were also identified. Analysis of the spatial distribution of diatoms and water quality characteristics within Florida Bay suggest that these assemblages may be structured by salinity and nutrient availability, particularly P. The Gulf of Mexico and the western Florida Bay assemblages were associated with higher water column salinities and TP concentrations and lower DIN concentrations and TN:TP ratios relative to the eastern Florida Bay assemblage. The temporal variation in diatom assemblages was associated with water temperature, though temporal indicator species were few relative to the number of spatial indicators.