Nutrient Over-enrichment Research Articles

Records of bulk organic matter and plant pigments in sediment of the “red-tide zone” adjacent to the Changjiang River estuary

Cultural eutrophication caused by nutrient over-enrichment in coastal waters will lead to a cascading set of ecosystem changes and deleterious ecological consequences, such as harmful algal blooms (HABs) and hypoxia. During the past two decades since the late 1990s, recurrent large-scale HABs (red tides) and an extensive hypoxic zone have been reported in the coastal waters adjacent to the Changjiang River estuary. To retrieve the history of eutrophication and its associated ecosystem changes, a sediment core was collected from the “red-tide zone” adjacent to the Changjiang River estuary. The core was dated using the 210Pb radioisotope and examined for multiple proxies, including organic carbon (OC), total nitrogen (TN), stable isotopes of C and N, and plant pigments. An apparent up-core increase of OC content was observed after the 1970s, accompanied by a rapid increase of TN. The concurrent enrichment of δ13C and increase of the C/N ratio suggested the accumulation of organic matter derived from marine primary production during this stage. The accumulation of OC after the 1970s well reflected the significant increase of primary production in the red-tide zone and probably the intensification of hypoxia as well. Plant pigments, including chlorophyll a, β-carotene, and diatoxanthin, showed similar patterns of variation to OC throughout the core, which further confirmed the important contribution of microalgae, particularly diatoms, to the deposited organic matter. Based on the variant profiles of the pigments representative of different microalgal groups, the potential changes of the phytoplankton community since the 1970s were discussed.

Nutrient enrichment intensifies hurricane impact in scrub mangrove ecosystems in the Indian River Lagoon, Florida, USA.

Mangroves are an ecological assemblage of trees and shrubs adapted to grow in intertidal environments along tropical, subtropical, and warm temperate coasts. Despite repeated demonstrations of their ecologic and economic value, multiple stressors including nutrient over-enrichment threaten these and other coastal wetlands globally. These ecosystems will be further stressed if tropical storm intensity and frequency increase in response to global climate changes. These stressors will likely interact, but the outcome of that interaction is uncertain. Here, we examined potential interaction between nutrient over-enrichment and the September 2004 hurricanes. Hurricanes Frances and Jeanne made landfall along Florida's Indian River Lagoon and caused extensive damage to a long-term fertilization experiment in a mangrove forest, which previously revealed that productivity was nitrogen (N) limited across the forest and, in particular, that N enrichment dramatically increased growth rates and aboveground biomass of stunted Avicennia germinans trees in the interior scrub zone. During the hurricanes, these trees experienced significant defoliation with three to four times greater reduction in leaf area index (LAI) than control trees. Over the long-term, the +N scrub trees took four years to recover compared to two years for controls. In the adjacent fringe and transition zones, LAI was reduced by > 70%, but with no differences based on zone or fertilization treatment. Despite continued delayed mortality for at least five years after the storms, LAI in the fringe and transition returned to pre-hurricane conditions in two years. Thus, nutrient over-enrichment of the coastal zone will increase the productivity of scrub mangroves, which dominate much of the mangrove landscape in Florida and the Caribbean; however, that benefit is offset by a decrease in their resistance and resilience to hurricane damage that has the potential to destabilize the system.

Effects of co-varying diel-cycling hypoxia and pH on disease susceptibility in the eastern oyster Crassostrea virginica

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 538:169-183 (2015) - DOI: https://doi.org/10.3354/meps11479 Effects of co-varying diel-cycling hypoxia and pH on disease susceptibility in the eastern oyster Crassostrea virginica Andrew G. Keppel1,2,*, Denise L. Breitburg1, Gary H. Wikfors3, Rebecca B. Burrell1, Virginia M. Clark1,2 1Smithsonian Environmental Research Center, Edgewater, MD 21037, USA 2Marine Estuarine and Environmental Sciences, University of Maryland, College Park, MD 20742, USA 3NOAA Fisheries Service, Northeast Fisheries Science Center, Milford, CT 06460, USA *Corresponding author: keppela@si.edu ABSTRACT: Diel-cycling hypoxia co-occurs with diel-cycling pH in shallow waters that are typically considered as refuge from deep-water hypoxia and are, therefore, targeted for restoration. These areas also tend to be heavily impaired by eutrophication from nutrient over-enrichment which increases the occurrence and severity of hypoxia and pH cycles. We used laboratory experiments to investigate the effects of diel-cycling dissolved oxygen (DO) and co-varying diel-cycling pH on infections of Perkinsus spp. and hemocyte activity in the eastern oyster Crassostrea virginica. Perkinsus marinus is the protistan parasite that causes Dermo disease in oysters. Severe diel-cycling DO increased the acquisition and progression of Perkinsus infections during exposure, and had a legacy effect the next year. Diel-cycling pH did not significantly affect infection dynamics either on its own or in combination with diel-cycling DO. Diel-cycling DO and pH both individually and in conjunction stimulated hemocyte activity, although this stimulated activity may not be effective at preventing Perkinsus infection. The magnitude of cycling conditions is an important consideration when choosing restoration sites, as severe cycling may hinder the re-establishment of oysters by creating areas that serve as reservoirs for parasites that can infect nearby populations. KEY WORDS: Hypoxia · pH · Diel cycle · Multiple stressors · Disease · Oyster Full text in pdf format Erratum PreviousNextCite this article as: Keppel AG, Breitburg DL, Wikfors GH, Burrell RB, Clark VM (2015) Effects of co-varying diel-cycling hypoxia and pH on disease susceptibility in the eastern oyster Crassostrea virginica. Mar Ecol Prog Ser 538:169-183. https://doi.org/10.3354/meps11479 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 538. Online publication date: October 28, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Long-Term Trends of Nutrients and Phytoplankton in Chesapeake Bay

Climate effects on hydrology impart high variability to water-quality properties, including nutrient loadings, concentrations, and phytoplankton biomass as chlorophyll-a (chl-a), in estuarine and coastal ecosystems. Resolving long-term trends of these properties requires that we distinguish climate effects from secular changes reflecting anthropogenic eutrophication. Here, we test the hypothesis that strong climatic contrasts leading to irregular dry and wet periods contribute significantly to interannual variability of mean annual values of water-quality properties using in situ data for Chesapeake Bay. Climate effects are quantified using annual freshwater discharge from the Susquehanna River together with a synoptic climatology for the Chesapeake Bay region based on predominant sea-level pressure patterns. Time series of water-quality properties are analyzed using historical (1945–1983) and recent (1984–2012) data for the bay adjusted for climate effects on hydrology. Contemporary monitoring by the Chesapeake Bay Program (CBP) provides data for a period since mid-1984 that is significantly impacted by anthropogenic eutrophication, while historical data back to 1945 serve as historical context for a period prior to severe impairments. The generalized additive model (GAM) and the generalized additive mixed model (GAMM) are developed for nutrient loadings and concentrations (total nitrogen—TN, nitrate + nitrate—NO2 + NO3) at the Susquehanna River and water-quality properties in the bay proper, including dissolved nutrients (NO2 + NO3, orthophosphate—PO4), chl-a, diffuse light attenuation coefficient (K D (PAR)), and chl-a/TN. Each statistical model consists of a sum of nonlinear functions to generate flow-adjusted time series and compute long-term trends accounting for climate effects on hydrology. We present results identifying successive periods of (1) eutrophication ca. 1945–1980 characterized by approximately doubled TN and NO2 + NO3 loadings, leading to increased chl-a and associated ecosystem impairments, and (2) modest decreases of TN and NO2 + NO3 loadings from 1981 to 2012, signaling a partial reversal of nutrient over-enrichment. Comparison of our findings with long-term trends of water-quality properties for a variety of estuarine and coastal ecosystems around the world reveals that trends for Chesapeake Bay are weaker than for other systems subject to strenuous management efforts, suggesting that more aggressive actions than those undertaken to date will be required to counter anthropogenic eutrophication of this valuable resource.

Environmental response of an Irish estuary to changing land management practices

Anthropogenic pressures have led to problems of nutrient over-enrichment and eutrophication in estuarine and coastal systems on a global scale. Recent improvements in farming practices, specifically a decrease in fertiliser application rates, have reduced nutrient loadings in Ireland. In line with national and European Directives, monitoring of Irish estuarine systems has been conducted for the last 30years, allowing a comparison of the effectiveness of measures undertaken to improve water quality and chemical and biological trends. The Blackwater Estuary, which drains a large agricultural catchment on the south coast of Ireland, has experienced a decrease in calculated nitrogen (N) (17%) and phosphorus (P) (20%) loads in the last decade. Monitored long-term river inputs reflect the reductions while estuarine P concentrations, chlorophyll and dissolved oxygen saturation show concurrent improvement. Consistently high N concentrations suggest a decoupling between N loads and estuarine responses. This highlights the complex interaction between N and P load reductions, and biochemical processes relating to remineralisation and primary production which can alter the effectiveness of the estuarine filter in reducing nutrient transport to the coastal zone. Effective management and reduction of both diffuse and point nutrient sources to surface waters require a consideration of the processes which may alter the effectiveness of measures in estuarine and coastal waters.

Tracking long-term human impacts on landscape, vegetal biodiversity and water quality in the Lake Aydat catchment (Auvergne, France) using pollen, non-pollen palynomorphs and diatom assemblages

Palaeoenvironmental studies allow the assessment of long-term human–climate–environmental interactions, and furnish valuable tools for the sustainable management of lacustrine ecosystems. A good example is the multi-proxy study of Lake Aydat's 19 m sedimentary core. Previous research revealed the role of climate and human activities on lake sedimentation, and identified two sedimentary units (6700 ± 200 to 3180 ± 90 and 1770 ± 60 cal. yr BP to present) separated by an erosive mass-wasting deposit (Lavrieux et al., 2013a). Pollen, non-pollen palynomorphs (e.g. fungal and algal spores, rotifer resting eggs), and diatom-based trophic reconstructions have been used to track the impacts of past land use on landscape evolution, vegetal biodiversity and water quality. Palaeoenvironmental data were also compared with local archaeo-historical datasets which allowed refined landscape reconstructions, especially for late Antiquity. The results obtained demonstrate that even Neolithic and Bronze Age human activities (between ca. 4600 and 4300 cal. yr BP and between ca. 3900 and 3500 cal. yr BP) had a discernible influence on catchment vegetation and lacustrine trophic dynamics of Lake Aydat, underlining the vulnerability of the ecosystem. Recurrent and complex models of past vegetation changes, phases of water nutrient over-enrichment and lake resilience were identified and related to grazing activities, but also to land use practises, which have been overlooked in Auvergne, such as mountain agriculture and hemp retting.

Climate effects on phytoplankton floral composition in Chesapeake Bay

Long-term data on floral composition of phytoplankton are presented to document seasonal and inter-annual variability in Chesapeake Bay related to climate effects on hydrology. Source data consist of the abundances of major taxonomic groups of phytoplankton derived from algal photopigments (1995–2004) and cell counts (1985–2007). Algal photopigments were measured by high-performance liquid chromatography (HPLC) and analyzed using the software CHEMTAX to determine the proportions of chlorophyll-a (chl-a) in major taxonomic groups. Cell counts determined microscopically provided species identifications, enumeration, and dimensions used to obtain proportions of cell volume (CV), plasma volume (PV), and carbon (C) in the same taxonomic groups. We drew upon these two independent data sets to take advantage of the unique strengths of each method, using comparable quantitative measures to express floral composition for the main stem bay. Spatial and temporal variability of floral composition was quantified using data aggregated by season, year, and salinity zone. Both time-series were sufficiently long to encompass the drought–flood cycle with commensurate effects on inputs of freshwater and solutes. Diatoms emerged as the predominant taxonomic group, with significant contributions by dinoflagellates, cryptophytes, and cyanobacteria, depending on salinity zone and season. Our analyses revealed increased abundance of diatoms in wet years compared to long-term average (LTA) or dry years. Results are presented in the context of long-term nutrient over-enrichment of the bay, punctuated by inter-annual variability of freshwater flow that strongly affects nutrient loading, chl-a, and floral composition. Statistical analyses generated flow-adjusted diatom abundance and showed significant trends late in the time series, suggesting current and future decreases of nutrient inputs may lead to a reduction of the proportion of biomass comprised by diatoms in an increasingly diverse flora.

Nutrient limitation dynamics examined on a multi-annual scale in Lake Taihu, China: implications for controlling eutrophication and harmful algal blooms

Rapidly increasing urban, agricultural, and industrial growth in the Taihu basin during the past four decades has led to accelerated nitrogen (N) and phosphorus (P) loading to the lake. This has caused the lake to shift from oligo-mesotrophic to hypertrophic conditions, symptomized by toxic cyanobacterial blooms, dominated by the non-N2 fixing genus Microcystis. From 2008 to 2013, a series of in situ microcosm and mesocosm nutrient addition bioassays were conducted that were focused on the heavily polluted northern region (i.e., Meiliang Bay) and other lake locations. Bioassays showed that phytoplankton production, as chlorophyll a and photopigments diagnostic of major phytoplankton groups, was controlled by P inputs from spring to early summer, while N played a more dominant controlling role in summer–fall. In most cases, combined N and P additions promoted maximum growth. This pattern proved true for both the highly eutrophic northern region and the less-eutrophic central and southern regions. Cyanobacteria, chlorophytes, and cryptophytes all showed the strongest positive responses to N and N+P enrichment during the summer bloom period, while diatoms were the least abundant then and just moderately stimulated by nutrient additions. Cyanobacteria failed to selectively respond to P inputs during the summer bloom period, contradicting the paradigm that selective P enrichment will favor them, especially the N2-fixing genera. Rather, Microcystis-dominated blooms remained N-limited during summer months and were not replaced by N2-fixing genera, indicating that internal N and P regeneration of previously loaded nutrients must be sustaining blooms. Successful ‘de-eutrophication’ of Taihu will require reductions of both N and P inputs in all lake regions in order to control blooms and counter the legacy of several decades of nutrient over-enrichment.

Determining critical nutrient thresholds needed to control harmful cyanobacterial blooms in eutrophic Lake Taihu, China.

Nutrient overenrichment has led to dramatic increases in harmful cyanobacterial blooms, creating serious threats to drinking water supplies, ecological and economic sustainability of freshwater ecosystems. Nutrient-cyanobacterial bloom interactions were examined in eutrophic Lake Taihu, China. In situ microcosm nutrient dilution bioassays and mesocosm nutrient addition experiments were conducted to determine nitrogen (N) and phosphorus (P) concentration and load thresholds needed to control cyanobacterial bloom formation. Blooms were dominated by toxic, non N2 fixing Microcystis spp, from May to December. Dilution bioassays showed seasonality in nutrient limitation, with P-availability controlling prebloom spring conditions and N-availability controlling summer-fall blooms. Nutrient dilution and enrichment bioassays indicated that total nitrogen (TN) and total phosphorus (TP) concentration thresholds should be targeted at below 0.80 mg L(–1) and 0.05 mg L(–1), respectively, to limit intrinsic growth rates of Microcystis dominated blooms. Based on estimates of nutrient loading and observed stoichiometry of phytoplankton biomass, 61–71% TN and 20–46% TP reduction are necessary to bring Taihu’s phytoplankton biomass to “acceptable” sub-bloom conditions of less than 20 μg L(–1) chlorophyll a.

Toward ecosystem-based management (EBM) of the world׳s large marine ecosystems during climate change

The approach to the assessment and management of Large Marine Ecosystems (LMEs) has been the subject of a series of seminal symposia convened at the annual meetings of the American Association for the Advancement of Science. Since 1995, a five module approach to ecosystem-based management (EBM) of LMEs has been the focus of 110 economically developing countries around the globe engaged in 22 LMEs. A sum of $3.1 billion in financial assistance from the Global Environment Facility and the World Bank has been provided to support this global movement towards recovery and sustainability of LME goods and services in partnership with several OECD countries, five UN agencies, and two NGOs. LME stressors of prime concern are nutrient overenrichment and climate warming stress from projected levels of reduced primary productivity in LMEs located between 30°N and 30°S. Successful mitigation actions for reducing nutrient overenrichment in LMEs, and adaptive precautionary actions for protecting fish and fisheries in LMEs in the high-risk circumglobal belt are addressed. The paper concludes with comments on the unity of approach in the linking of science and policy in advancing toward sustainability of the world’s LMEs in accordance with Rio+20 goals.

Silica uptake by Spartina-evidence of multiple modes of accumulation from salt marshes around the world.

Silicon (Si) plays a critical role in plant functional ecology, protecting plants from multiple environmental stressors. While all terrestrial plants contain some Si, wetland grasses are frequently found to have the highest concentrations, although the mechanisms driving Si accumulation in wetland grasses remain in large part uncertain. For example, active Si accumulation is often assumed to be responsible for elevated Si concentrations found in wetland grasses. However, life stage and differences in Si availability in the surrounding environment also appear to be important variables controlling the Si concentrations of wetland grasses. Here we used original data from five North American salt marshes, as well as all known published literature values, to examine the primary drivers of Si accumulation in Spartina, a genus of prolific salt marsh grasses found worldwide. We found evidence of multiple modes of Si accumulation in Spartina, with passive accumulation observed in non-degraded marshes where Spartina was native, while rejective accumulation was found in regions where Spartina was invasive. Evidence of active accumulation was found in only one marsh where Spartina was native, but was also subjected to nutrient over-enrichment. We developed a conceptual model which hypothesizes that the mode of Si uptake by Spartina is dependent on local environmental factors and genetic origin, supporting the idea that plant species should be placed along a spectrum of Si accumulation. We hypothesize that Spartina exhibits previously unrecognized phenotypic plasticity with regard to Si accumulation, allowing these plants to respond to changes in marsh condition. These results provide new insight regarding how salt marsh ecosystems regulate Si exchange at the land-sea interface.

Hypoxia sustains cyanobacteria blooms in the Baltic sea.

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000–4000 B.P. during the Littorina transgression, (2) ca. 1400–700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions.

Adaptive management institutions at the regional level: The case of Large Marine Ecosystems

A global effort is underway by scientists, stakeholders, resource managers, and multisectoral ministerial representatives (e.g. fisheries, transportation, mining, energy, tourism, environment) from 110 economically developing countries to implement ecosystem-based management at the Large Marine Ecosystem scale. The effort is supported with $3.1 billion in financial assistance from the Global Environmental Facility and World Bank to assess and manage goods and services of Large Marine Ecosystems (LMEs) along the coasts of economically developing countries in Africa, Asia, Latin America and eastern Europe. Through a systematic spatial and temporal scaling across multiple jurisdictions (e.g. community, municipal, regional, national, and international) a generic suite of indicators is applied to monitor the annual changes in LME productivity, fish and fisheries, pollution and ecosystem health, socioeconomics, and governance. Ecosystem-based governance practices are being implemented by Commissions that serve as institutional frameworks for restoring and sustaining transboundary LME goods and services. Under activities guided by LME Commissions, the suites of indicators are analyzed in relation to drivers of change and the results are applied to adaptive management regimes to reduce coastal pollution, restore damaged habitats, recover depleted fisheries conserve biodiversity, control nutrient over-enrichment and ocean acidification, and mitigate and adapt to the effects of climate warming. Application of ecosystem-based adaptive management practices presently underway by the People's Republic of China and the Republic of Korea are discussed for the Yellow Sea LME.

Latitudinal Patterns of Herbivory in Mangrove Forests: Consequences of Nutrient Over-Enrichment

Ecosystems in the tropics are predicted to have stronger responses to nutrient enrichment, greater diversity, and more intense biotic interactions than in temperate areas. Mangrove forests, which occur across a broad biogeographic range from warm temperate to tropical, provide a unique opportunity to test these hypotheses by investigating the responses of herbivores to nutrient enrichment in temperate versus tropical latitudes. Mangroves are complex intertidal ecosystems with spatial differences in structure and diversity along tidal gradients and are threatened globally by human activities including nutrient over-enrichment. In this study, we used long-term fertilization experiments at the Indian River Lagoon, FL; Twin Cays, Belize; and Bocas del Toro, Panama to determine how increased nutrients impact herbivore abundance and herbivory of Rhizophora mangle at the tree, forest, and regional scales. At these locations, which span approximately 2185 km and 18.4o of latitude, we fertilized individual trees with one of three treatments (Control, +N, +P) in two zones (fringe, scrub) along transects perpendicular to the shoreline and measured their responses for 4 years. Herbivory was measured as folivory, loss of yield, and tissue mining. Although nutrient enrichment altered plant growth, leaf traits, and nutrient dynamics, these variables had little effect on folivory at any location. Our results did not support the prediction that herbivory and per capita consumption are greatest at the most tropical location. Instead, folivory was highest at the most temperate location and lowest at the intermediate location. Folivory was generally higher in the fringe than in the scrub zone, but the pattern varied by location, herbivore, and nutrient treatment. Folivory by a dominant herbivore, Aratus pisonii, decreased from the highest to the lowest latitude. Our data suggest that factors controlling population dynamics of A. pisonii cascade to the mangrove canopy, linking herbivory to crab densities.

Changing states of North Atlantic large marine ecosystems

Effects of climate forcing are examined for 15 large marine ecosystems bordering the North Atlantic basin. Trends in multi-decadal time-series data of temperature, chlorophyll, primary productivity, nutrients, and fisheries yields, differed among the LMEs. Responses to climate warming varied between northwestern and northeastern Atlantic LMEs, with warming rates influencing changes in northeast Atlantic LME plankton production and fisheries yields more directly than in LMEs of the northwest Atlantic, where warming rates are lower. In contrast, negative effects of nutrient over-enrichment in relation to harmful algal blooms and hypoxic conditions were greater in the northwest Atlantic LMEs. Forecasts suggest significant increases in nutrient over-enrichment of North Atlantic LMEs by 2050. Fishery time-series analyses suggest increases in fishery yields for sub-arctic LMEs, and declines in LMEs of more southerly latitudes.

Relationship between diatoms and water quality (TN, TP) in sub-tropical east Australian estuaries

Nutrient over-enrichment of estuarine environments is increasing globally. However, it is difficult to determine the eutrophication trend in estuaries over long periods of time because long-term monitoring records are scarce and do not permit the identification of baseline environmental conditions. In this study, preliminary diatom based transfer functions for the inference of total phosphorus (TP) and total nitrogen (TN) in east-Australian sub-tropical estuaries were developed to address the deficiency in knowledge relating to historical estuary water quality trends. The transfer functions were created from a calibration set consisting of water quality and associated surface sediment diatom assemblage data from fifty-two sub-tropical estuaries in New South Wales and Queensland, Australia. Following data screening processes, Canonical Correspondence Analysis confirmed that TP and TN both explained significant, independent variation in the diatom assemblages. Variance partitioning, however, indicated that the TP was confounded with and may receive some strength from TN. WA and WA-PLS 2 component models for TP that included all calibration set sites yielded statistically weak results based on the jack-knifed r 2 scores $$ \left( {r_{\text{jack}}^{{^{ 2} }} \, = 0.22\;{\text{and}}\;0. 2 2 {\text{ respectively}}} \right) $$ . Removal from the calibration set of 12 sites that had all PO4, NH4, NO2, and NOx concentrations below detection limit resulted in a substantial improvement in WA-PLS 2 component TP model scores $$ \left( {r_{\text{jack}}^{{^{ 2} }} \; = \;\,0.69} \right) $$ , indicating that this model is statistically robust, and thus suitable for down core nutrient reconstructions. Caution, however, is required when developing diatom based inference models in Australian estuaries as nutrient cycling processes may have the potential to influence diatom based transfer functions. The model reported on here provides a foundation for reconstructing nutrient histories in eastern Australian sub-tropical estuaries in the absence of monitoring data.

Harmful Cyanobacterial Blooms: Causes, Consequences, and Controls

Cyanobacteria are the Earth's oldest oxygenic photoautotrophs and have had major impacts on shaping its biosphere. Their long evolutionary history (≈ 3.5 by) has enabled them to adapt to geochemical and climatic changes, and more recently anthropogenic modifications of aquatic environments, including nutrient over-enrichment (eutrophication), water diversions, withdrawals, and salinization. Many cyanobacterial genera exhibit optimal growth rates and bloom potentials at relatively high water temperatures; hence global warming plays a key role in their expansion and persistence. Bloom-forming cyanobacterial taxa can be harmful from environmental, organismal, and human health perspectives by outcompeting beneficial phytoplankton, depleting oxygen upon bloom senescence, and producing a variety of toxic secondary metabolites (e.g., cyanotoxins). How environmental factors impact cyanotoxin production is the subject of ongoing research, but nutrient (N, P and trace metals) supply rates, light, temperature, oxidative stressors, interactions with other biota (bacteria, viruses and animal grazers), and most likely, the combined effects of these factors are all involved. Accordingly, strategies aimed at controlling and mitigating harmful blooms have focused on manipulating these dynamic factors. The applicability and feasibility of various controls and management approaches is discussed for natural waters and drinking water supplies. Strategies based on physical, chemical, and biological manipulations of specific factors show promise; however, a key underlying approach that should be considered in almost all instances is nutrient (both N and P) input reductions; which have been shown to effectively reduce cyanobacterial biomass, and therefore limit health risks and frequencies of hypoxic events.

Coastal Zone Facing Climate Change: Knowledge Towards New Solutions

Humanity’s impact on the Earth system has become comparable to planetary-scale geological processes, such as ice ages. Consensus is growing that we have driven the planet into a new epoch, the Anthropocene, in which many Earth-system processes and the living fabric of ecosystems are now dominated by human activities. That the Earth has experienced large-scale, abrupt changes in the past indicates that it could experience similar changes in the future. This recognition has led researchers to take the first step to identify planetary and regional thresholds, and boundaries that, if crossed, could generate unacceptable environmental and social change [1]. Galvanized by the United Nations Conference on Environment and Development meeting of 1992, there has been a search for methods to build capacity of coastal nations and communities to manage their coastal and estuarine resources in a sustainable manner. Countries are now increasingly being affected mostly by hydro-meteorological hazards and disasters, namely, floods, mass movements (e.g. erosion, landslides and siltation), heat waves, wildfires, tropical cyclones, tsunamis, swells and dust storms [2,3]. Research now demonstrates that the continued functioning of the Earth system, as it has supported the well-being of human civilization in recent centuries is at risk. Without urgent action, we could face threats to water, food, biodiversity and other critical resources: these threats risk intensifying economic, ecological and social crises, creating the potential for a humanitarian emergency on a global scale. Nutrients are now the largest pollution problem in coastal marine ecosystems globally, and in the US, some 60% of estuaries are moderately to severely degraded, primarily from nutrients like nitrates and phosphates [4-7]. Developing economies tend to be dominated by resource-based activities with degrading and overexploitive uses of land, water and other coastal resources, with disruption of environmental processes through degradation of environmental quality and loss of critical terrestrial and aquatic habitats, leading to serious deleterious impacts on the health and productivity of coastal ecosystems. Studies have also showed that estuaries are particularly vulnerable to nutrient over-enrichment and trace metal contamination [8-12].

The “salt wedge pump”: Convection‐driven pore‐water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over‐enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high‐resolution radon (a natural groundwater and pore‐water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced pore‐water exchange at this area. We use the term “salt wedge pump” to describe convection‐driven advective pore‐water exchange at the sediment–water interface during the upstream propagation of the salt wedge. Radon‐derived convection‐driven pore‐water exchange rates within the salt wedge were estimated at 2.8 cm d−1, a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore‐water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes ∼ 10‐fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5‐fold higher than oxygen uptake related to advective pore‐water exchange. Estimated fluxes, associated with the nonconservative DIC, δ13C‐DIC, and ammonium behavior within the estuary support convective pore‐water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems.

Does increased model complexity improve description of phosphorus dynamics in a large treatment wetland?

As the structure of ecological models grows more complex, it becomes increasingly important to identify the appropriate level of complexity for reliable process description and prediction. Here, a suite of mechanistic biogeochemical models with different levels of complexity for representing phosphorus cycling processes was developed and tested against observations from a large treatment wetland. The study site was the 147-ha Cell 4 of Stormwater Treatment Area 1 West, which was designed to help protect the greater Everglades, FL, USA, from nutrient over-enrichment. Six biogeochemical models of differing complexity were coupled with a pre-calibrated two-dimensional hydrodynamic model of Cell 4 and tested against field data. We provide guidance for evaluating a set of models with varying level of complexity using key model attributes that influence the suitability of a model or a set of models. Considerations of model accuracy, complexity, and explanatory depth are combined into a single indicator of model effectiveness. Results revealed that the most complex model structure may not necessarily be the most effective in simulating the dynamics of total phosphorus (TP) concentrations in the wetland. The rate of improvement in the model performance decreased as model complexity increased. Although the most complex model reproduced the field observations best, the marginal improvement in model performance compared to simpler models was outweighed by the higher costs of increased complexity. Highly detailed representations of system structures may not be useful to simulate TP dynamics in treatment wetlands if comprehensive data sets are not available to constrain each pathway. It is crucial for model developers and users to evaluate model structures of differing complexity to identify the appropriate level of complexity for given data and questions of interest.