Functioning Of Estuarine Ecosystems Research Articles

Habitat-specific distinctions in estuarine denitrification affect both ecosystem function and services

Resource limitation controls the base of food webs in many aquatic ecosystems. In coastal ecosystems, nitrogen (N) has been found to be the predominant limiting factor for primary producers. Due to the important role nitrogen plays in determining ecosystem function, understanding the processes that modulate its availability is critical. Shallow-water estuarine systems are highly heterogeneous. In temperate estuaries, multiple habitat types can exist in close proximity to one another, their distribution controlled primarily by physical energy, tidal elevation and geomorphology. Distinctions between these habitats such as rates of primary productivity and sediment characteristics likely affect material processing. We used membrane inlet mass spectrometry to measure changes in N2 flux (referred to here as denitrification) in multiple shallow-water estuarine habitats through an annual cycle. We found significantly higher rates of denitrification (DNF) in structured habitats such as submerged aquatic vegetation, salt marshes and oyster reefs than in intertidal and subtidal flats. Seasonal patterns were also observed, with higher DNF rates occurring in the warmer seasons. Additionally, there was an interaction between habitat type and season that we attributed to the seasonal patterns of enhanced productivity in individual habitat types. There was a strong correlation between denitrification and sediment oxygen demand (SOD) in all habitats and all seasons, suggesting the potential to utilize SOD to predict DNF. Denitrification efficiency was also higher in the structured habitats than in the flats. Nitrogen removal by these habitats was found to be an important contributor to estuarine ecosystem function. The ecosystem service of DNF in each habitat was evaluated in US dollars using rates from a regional nutrient-offset market to determine the cost to replace N through management efforts. Habitat-specific values of N removal ranged from approximately three thousand U.S. dollars per acre per year in the submerged aquatic vegetation to approximately four hundred U.S. dollars per acre per year in the subtidal flat. Because of the link between habitat type and processes such as DNF, changes in habitat area and distribution will have consequences for both ecosystem function and the delivery of ecosystem services.

Estuarine microbial community characteristics as indicators of human-induced changes (Senegal River, West Africa)

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L −1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l −1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 10 4 cells l −1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

The endocrine control of reproduction in Nereidae: a new multi-hormonal model with implications for their functional role in a changing environment

Nereidae are vital to the functioning of estuarine ecosystems and are major components in the diets of over-wintering birds and commercial fish. They use environmental cues to synchronize reproduction. Photoperiod is the proximate cue, initiating vitellogenesis in a temperature-compensated process. The prevailing paradigm in Nereidae is of a single 'juvenile' hormone controlling growth and reproduction. However, a new multi-hormone model is presented here that integrates the environmental and endocrine control of reproduction. This is supported by evidence from in vitro bioassays. The juvenile hormone is shown to be heat stable and cross reactive between species. In addition, a second neuro-hormone, identified here as a gonadotrophic hormone, is shown to be present in mature females and is found to promote oocyte growth. Furthermore, dopamine and melatonin appear to switch off the juvenile hormone while serotonin and oxytocin promote oocyte growth. Global warming is likely to uncouple the phase relationship between temperature and photoperiod, with significant consequences for Nereidae that use photoperiod to cue reproduction during the winter in northern latitudes. Genotypic adaptation of the photoperiodic response may be possible, but significant impacts on fecundity, spawning success and recruitment are likely in response to short-term extreme events. Endocrine-disrupting chemicals may also impact on putative steroid hormone pathways in Nereidae with similar consequences. These impacts may have significant implications for the functional role of Nereidae and highlight the importance of comparative endocrinology studies in these and other invertebrates.

Estuarine Phytoplankton Responses to Hurricanes and Tropical Storms with Different Characteristics (Trajectory, Rainfall, Winds)

We examined the short-term (<1 month post-storm) impact of storms [Tropical Storm (TS) Helene in 2000, Hurricane (H) Isabel in 2003, H Alex, Tropical Depression (TD) Bonnie and TS Charley in 2004] varying in their trajectory, wind and rainfall characteristics, on water column structure, nutrients, and phytoplankton biomass in North Carolina’s Neuse R. Estuary (NRE). Data are presented from two sampling programs, ModMon (biweekly) and FerryMon (measurements made every 3 min daily). Helene’s winds mixed the previously stratified water column, delivering sediment-bound nutrients to the euphotic zone, and localized freshwater input from Helene was also evident. Mean chlorophyll a concentrations in the mesohaline portion of the NRE, where N was strongly limiting before the storm (molar DIN:DIP < 1), more than doubled after the storm. Unlike with Helene, the water column was well mixed before passage of Isabel, and nutrient concentrations were high. As a result, minimal impact on phytoplankton biomass was detected despite Isabel’s high winds and significant freshwater input. In fact, conditions became less favorable for phytoplankton growth after the storm. Alex was fast moving and relatively small, but its winds were sufficient to mix the water column. Although data from ModMon suggest that chlorophyll a was only slightly higher after passage of Alex, FerryMon detected an ephemeral bloom that was missed by ModMon. Overall, these results suggest that relatively small tropical storms and hurricanes can lead to significant increases in phytoplankton biomass. However, the phytoplankton response depends on both the characteristics of a particular storm and the physical–chemical conditions of the water column before storm passage. Finally, the ephemeral bloom that developed as a result of Alex, the strong response of phytoplankton in the mesohaline portion of the estuary to nutrient inputs, and their patchiness on several other occasions suggests that storms may create “hot spots” for trophic transfer and biogeochemical dynamics in estuaries. Adaptive sampling is necessary to capture these features and to fully understand the impact of perturbations such as storms on estuarine ecosystem functioning.

Ecosystem engineering by annual intertidal seagrass beds: Sediment accretion and modification

Seagrasses are generally known as ecosystem engineers, as they reduce flow velocities in their canopies. In perennial subtidal meadows, this usually leads to increased net sedimentation rates and reduction of the grain size. The present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics. Sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat. Within the planting units (79 shoots m −2) 4.7 mm of sediment accreted, whereas in the most dense parts of these units (199 shoots m −2) accretion amounted to 7.1 mm. The silt fraction (<63 μm) increased and the sand fraction (63–500 μm) decreased in the eelgrass beds, which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass presence. Annual intertidal eelgrass beds significantly contribute to the immobilisation of sediment during the growing season with its magnitude depending on canopy density. During winter, the accumulated sediments were released again and could even induce additional erosion. Possible consequences of these sediment dynamics for the larger scale functioning of estuarine ecosystems are discussed.

Mangrove growth in New Zealand estuaries: the role of nutrient enrichment at sites with contrasting rates of sedimentation

Mangrove forest coverage is increasing in the estuaries of the North Island of New Zealand, causing changes in estuarine ecosystem structure and function. Sedimentation and associated nutrient enrichment have been proposed to be factors leading to increases in mangrove cover, but the relative importance of each of these factors is unknown. We conducted a fertilization study in estuaries with different sedimentation histories in order to determine the role of nutrient enrichment in stimulating mangrove growth and forest development. We expected that if mangroves were nutrient-limited, nutrient enrichment would lead to increases in mangrove growth and forest structure and that nutrient enrichment of trees in our site with low sedimentation would give rise to trees and sediments that converged in terms of functional characteristics on control sites in our high sedimentation site. The effects of fertilizing with nitrogen (N) varied among sites and across the intertidal zone, with enhancements in growth, photosynthetic carbon gain, N resorption prior to leaf senescence and the leaf area index of canopies being significantly greater at the high sedimentation sites than at the low sedimentation sites, and in landward dwarf trees compared to seaward fringing trees. Sediment respiration (CO(2) efflux) was higher at the high sedimentation site than at the low one sedimentation site, but it was not significantly affected by fertilization, suggesting that the high sedimentation site supported greater bacterial mineralization of sediment carbon. Nutrient enrichment of the coastal zone has a role in facilitating the expansion of mangroves in estuaries of the North Island of New Zealand, but this effect is secondary to that of sedimentation, which increases habitat area and stimulates growth. In estuaries with high sediment loads, enrichment with N will cause greater mangrove growth and further changes in ecosystem function.

Effects of Spartina alterniflora invasion on benthic nematode communities in the Yangtze Estuary

Invasions of exotic plant species in estuaries have occurred worldwide, and may pro- foundly affect native biodiversity and estuarine ecosystem functioning. To assess the effect of plant invasions on benthic meiofauna, we compared the nematode communities in marshes dominated, respectively, by invasive Spartina alterniflora and native Scirpus mariqueter and Phragmites australis at 3 local sites over 2 seasons in the Yangtze River estuary, PR China. S. alterniflora stands had gen- erally lower nematode trophic diversity than the stands of the 2 native plants, suggesting that the exotic plant led to a simplified benthic food web. The relative abundance of bacterial-feeding nema- todes tended to increase in S. alterniflora marshes compared to P. australis marshes. The increased bacterial-feeding nematodes in S. alterniflora stands are likely to reflect the altered decomposition processes, rates and pathways, which may, in turn, modify belowground nutrient cycling of the estu- arine ecosystems. The dissimilarity in nematode community structure between S. alterniflora and S. mariqueter marshes was smaller than that between S. alterniflora and P. australis marshes, and the dissimilarity between Spartina-invaded and native marshes was even smaller than between the 2 native plant marshes. It is suggested that the detection of the ecological consequences of plant inva- sions depends on which native plant species is considered. Site effects were generally detected in the comparison of sediment properties and nematodes among 3 plant marshes. Sediment water content, electrical conductivity, bacterial biomass and litter biomass were identified as the most important factors in the shaping of the nematode communities.

Possible effects of climate change on estuarine nutrient fluxes: a case study in the highly nutrified Schelde estuary (Belgium, The Netherlands)

Global change models predict effects of climate change on hydrological regimes at the continental scale in Europe. The aim of this study was to gain a better understanding of the possible effect of changing external forcing conditions on the functioning of estuarine ecosystems. In densely populated areas, anthropogenic nutrient enrichment and consequent alteration of nutrient biogeochemical cycles have already had a big impact on these ecosystems. The average yearly discharge of the upper Schelde estuary increased nearly threefold over the period 1996–2000, from 28 m 3 s −1 in 1996 to 73 m 3 s −1 in 2000. The continuously rising discharge conditions over the five-year period were used as a reference situation for possible future effects of climate on ecological functioning through increase of discharge. At high discharges, nutrient (NH 4 +, NO 3 −, dissolved silica and PO 4 3−) concentrations in the tidal fresh- and brackish water showed a decrease of up to 50% while total discharged nutrient loadings increased up to 100%. Opposite effects of increasing discharge on NH 4 +, NO 3 − and dissolved silica concentrations in summer and winter, resulted in the flattening out of seasonal cycles for these nutrients. Under high discharge conditions, silica uptake by diatom communities was lowered. Dissolved silica loadings to the coastal area increased concurrently with total silica loadings upstream. Salt intrusion to the marine parts of the estuary decreased. This resulted in a downstream shift of the salinity gradient, with lower salinity observed near the mouth. As a result, TDIN, NO 3 − and dissolved silica concentrations doubled at the mouth of the estuary.

Possible effects of climate change on estuarine nutrient fluxes: a case study in the highly nutrified Schelde estuary (Belgium, The Netherlands)

Global change models predict effects of climate change on hydrological regimes at the continental scale in Europe. The aim of this study was to gain a better understanding of the possible effect of changing external forcing conditions on the functioning of estuarine ecosystems. In densely populated areas, anthropogenic nutrient enrichment and consequent alteration of nutrient biogeochemical cycles have already had a big impact on these ecosystems. The average yearly discharge of the upper Schelde estuary increased nearly threefold over the period 1996–2000, from 28 m3 s−1 in 1996 to 73 m3 s−1 in 2000. The continuously rising discharge conditions over the five-year period were used as a reference situation for possible future effects of climate on ecological functioning through increase of discharge. At high discharges, nutrient (NH4+, NO3−, dissolved silica and PO43−) concentrations in the tidal fresh- and brackish water showed a decrease of up to 50% while total discharged nutrient loadings increased up to 100%. Opposite effects of increasing discharge on NH4+, NO3− and dissolved silica concentrations in summer and winter, resulted in the flattening out of seasonal cycles for these nutrients. Under high discharge conditions, silica uptake by diatom communities was lowered. Dissolved silica loadings to the coastal area increased concurrently with total silica loadings upstream. Salt intrusion to the marine parts of the estuary decreased. This resulted in a downstream shift of the salinity gradient, with lower salinity observed near the mouth. As a result, TDIN, NO3− and dissolved silica concentrations doubled at the mouth of the estuary.

Effects of fishing on the structure and functioning of estuarine and nearshore ecosystems

Estuaries and associated coastal waters support many essential fisheries, a fact which contributes to their disproportionately high economic value. They are, however, also among the most extensively modified and threatened of aquatic environments. Almost all have been strongly affected by human beings, and fisheries are an integral part of human activities on the coast. We have taken a global perspective in synthesizing the effects of fishing on estuaries and coastal waters. Rather than attempt to cover all regions of the world in detail, we review eight process-orientated categories affected by fishing, with case studies for each of them: target organisms, non-target organisms, nursery functions, trophic effects, habitat change, reduced water quality, human environment, and potential for local extinctions. Fishing in the estuarine and nearshore environment has clear impacts on the structure and functioning of these ecosystems, although other, non-fishing issues also effect these ecosystems. This creates multiple interactions and reinforces the need for an integrated approach to coastal zone management. Nonetheless, some form of fish-based action plan could be created, especially within estuaries, which would provide management objectives for a particular system.

HUMAN ALTERATION OF THE GLOBAL NITROGEN CYCLE: SOURCES AND CONSEQUENCES

Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental consequences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; increased concentrations of the potent greenhouse gas N2O globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; contributed substantially to the acidification of soils, streams, and lakes in several regions; and greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: increased the quantity of organic carbon stored within terrestrial ecosystems; accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.

Technical Report: Human Alteration of the Global Nitrogen Cycle: Sources and Consequences

Nitrogen is a key element controlling the species composition, diversity, dynamics, and functioning of many terrestrial, freshwater, and marine ecosystems. Many of the original plant species living in these ecosystems are adapted to, and function optimally in, soils and solutions with low levels of available nitrogen. The growth and dynamics of herbivore populations, and ultimately those of their predators, also are affected by N. Agriculture, combustion of fossil fuels, and other human activities have altered the global cycle of N substantially, generally increasing both the availability and the mobility of N over large regions of Earth. The mobility of N means that while most deliberate applications of N occur locally, their influence spreads regionally and even globally. Moreover, many of the mobile forms of N themselves have environmental consequences. Although most nitrogen inputs serve human needs such as agricultural production, their environmental consequences are serious and long term. Based on our review of available scientific evidence, we are certain that human alterations of the nitrogen cycle have: 1) approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle, with these rates still increasing; 2) increased concentrations of the potent greenhouse gas N20 globally, and increased concentrations of other oxides of nitrogen that drive the formation of photochemical smog over large regions of Earth; 3) caused losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility; 4) contributed substantially to the acidification of soils, streams, and lakes in several regions; and 5) greatly increased the transfer of nitrogen through rivers to estuaries and coastal oceans. In addition, based on our review of available scientific evidence we are confident that human alterations of the nitrogen cycle have: 6) increased the quantity of organic carbon stored within terrestrial ecosystems; 7) accelerated losses of biological diversity, especially losses of plants adapted to efficient use of nitrogen, and losses of the animals and microorganisms that depend on them; and 8) caused changes in the composition and functioning of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries. Manuscript received 1 November 1996. Reprints of this 14-page report are available for $2.25 each. Prepayment is required. Order reprints from the Ecological Society of America, Attention: Reprint Department, 2010 Massachusetts Avenue, NW, Suite 400, Washington, D.C. 20036.

The mutual relationship between the monitoring and modelling of estuarine ecosystems

A general simulation model developed for the Ems estuary (a part of the Wadden Sea), to describe the main carbon flow through the foodweb, was applied for monitoring purposes. In this model, the estuary is divided into five compartments, in each of which a pelagic, an epibenthic and a benthic submodel operates. A transport model regulates the distribution of dissolved and particulate substances over the area. Two parameters that partly regulate the carbon flow are the concentrations of phosphate and of the particles (turbidity) in the water. From the sensitivity analyses for phosphate and turbidity it can be concluded that the relative effects of changes in turbidity and changes in the influx of phosphate from the rivers are large in the upper reaches and small in the lowest reaches of the estuary. This is due to the increasing influence of the coastal waters in the lower reaches. These results indicate that the reaches, which represent the beginning and the end of a gradient in the estuary, are also very suitable for monitoring the quality status of this estuary in terms of production and standing stock of groups of organisms. The need to use monitored data from the boundaries (sea and rivers) of those ecosystems as input in simulation models is discussed. Moreover, the possibilities of using simulation models to generate new ideas on the functioning of estuarine ecosystems under changing environmental conditions and to help administrators to decide on specific future management strategies are also discussed.

Estuarine, Coastal and Ocean Pollution: EEC Policy and Research

This paper presents an overview of the policy and research of the European Community concerning estuarine, coastal and marine pollution. The structure and implementation of the European Community marine environmental policy, intended to combat the various types of pollution of the sea, and to protect the European waterways and the marine environment are described. Particular reference is made to actions dealing with oil pollution and other harmful chemical substances released into the sea. The environmental research programme, aimed at providing scientific support to the environmental policy of the European Community, addresses short-, mid- and long-term problems. Thus it deals mostly with pollution : its sources, detection, fate, pathways, environmental and genetic effects; and abatement and prevention of pollution by oil, wastes, fertilizers, heavy metals and other environmental chemicals. It is also increasingly concerned with improving the understanding of the functioning of estuarine, coastal and marine ecosystems. The future orientations of the Community's marine environmental policy and research are discussed. These will stress the preventive aspects of environmental protection.