Levels Of Nutrient Loading Research Articles

Effects of different nutrient loadings on planktonic primary production in embayments of Indonesia

A study on planktonic primary production was conducted in three Indonesian embayments subjected to different levels of eutrophication. The three embayments are Jakarta, Lampung and Semangka Bay. A previous study revealed that Jakarta Bay is heavily eutrophied while Lampung Bay is moderately eutrophied and Semangka Bay is close to pristine conditions. The eutrophication state in the three bays studied was closely related with respective nutrient loading of the embayments. Annual primary production in Jakarta Bay ranged from 47 to 503 g C m-2 y-1, in Lampung Bay from 31 to 196 g C m-2 y-1 and in Semangka Bay from 14 to 40 g C m-2 y-1. Spatial gradients in primary productivity were closely related to spatial nutrient gradients in the bays, except in Semangka Bay, due to the high turbidity of the river mouth. Temporal variability in primary productivity was small, consistent with small changes in environmental parameters across seasons. Jakarta Bay which has the most eutrophied waters was the highest in terms of primary productivity, followed by Lampung Bay and Semangka Bay. The primary productivity levels in the three bays was closely related with levels of nutrient loadings. This research is of importance for the development of primary production measurement in Indonesia since it is among that uses the radiocarbon technique.

Growth, Nitrogen Uptake, and Nutritional Value of a Diverse Panel of Shrub Willow (Salix spp.) Genotypes in Response to Nitrogen Fertilization

Riparian buffers are a practical strategy to reduce N runoff. Shrub willows (Salix spp.) are a well-suited natural means to manage water quality due to dense aboveground biomass growth, diffuse root systems at a shallow depth, and low water- and nutrient-use efficiencies that will maximize uptake. Perennial forage crops in the margins of agricultural lands could provide biomass for fodder. The response of fifty genotypes to a single N level of fertilizer under standard conditions at a very high nutrient loading level (500 ppm N, delivered weekly) was compared with water-only controls. Irrigation with excess N caused greater aboveground growth measured as biomass, stem length, and diameter, as well as morphological properties that varied significantly by genotype, indicating a potential for selection in a breeding program. SPAD was a reliable indicator of the effect of fertilizer on foliar crude protein and showed different effects of fertilizer on N uptake and nutritional value among genotypes. We identified genotypes that display high N uptake and good relative feed value. This intentional design for environmental benefits could have a dual purpose should this potentially nutrient-dense biomass by used as fodder for small ruminants.

Eutrophication decreases Halophila beccarii plant organic carbon contribution to sequestration potential

Seagrass ecosystems rank among the most effective blue carbon sinks in climate change mitigation and greenhouse gas removal. Nutrient pollution has emerged as a leading threat to seagrass decline and has diminished the carbon sequestration potential in recent decades. Changes in the nutrient regime can also impact the organic carbon compositions (labile and refractory organic carbon compositions) of seagrass tissues, with important implications for determining the quantity and quality of carbon sequestration. However, there is still little information about the impact of nutrient loading on seagrass plant refractory organic carbon composition (ROC), which hinders our ability to reveal the driving mechanisms of anthropogenic factors that decrease seagrass organic carbon sequestration capability. Here, a multidisciplinary approach was employed to investigate the organic carbon variations of Halophila beccarii at five seagrass meadows with contrasting nutrient loading levels. The results showed that H. beccarii plant nitrogen (N) content ranged from 2.21% to 5.65%, which well reflected the external nutrient loading levels. High nutrient loading elevated labile organic carbon content, like free amino acids and soluble sugars. Nevertheless, ROC content (cellulose-associated organic matter) decreased with increasing nutrient loading, which presented a significant negative linear correlation with plant N content. These results provide evidence that eutrophic conditions enhance H. beccarii plant quality (high N and labile organic carbon) and consequently decrease plant ROC sequestration potential. This suggests that reducing nitrogen input to seagrass meadows would aid in increasing seagrass carbon storage.

极端天气陆地河流面源污染对珠江河口叶绿素分布的影响——评价方法构建与应用

PDF HTML阅读 XML下载 导出引用 引用提醒 极端天气陆地河流面源污染对珠江河口叶绿素分布的影响——评价方法构建与应用 DOI: 10.5846/stxb202103130681 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 南方海洋科学与工程广东省实验室（广州）人才团队引进重大专项（GML2019ZD0303）；热带海洋环境国家重点实验室自主研究项目（LTOZZ2103）；国家自然科学基金面上项目（42077057，41876136） Impacts of non-point source pollutants from river on chlorophyll distribution in the Pearl River Estuary under the extreme weather conditions: the framework and application Author: Affiliation: Fund Project: 100 Talents Program of the Chinese Academy of Sciences，The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan), Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) talent team introduced major special projects 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:近十年来，随着我国粤港澳大湾区的建设，珠江流域人口增长十分迅速，食品和能源需求急剧增加，导致流域上游省份农业用地面积增加，下游省份城市化进程加速，随之导致含农业化肥的农田灌溉用水和生活污水排放大幅度增加。流域的面源污染物进入珠江河口，导致河口海域赤潮发生频率明显增高。此外，全球变暖加剧了南海和西太台风等极端天气事件的发生频率，大量的台风在珠江口附近登陆，对人类活动产生的营养盐在海水中再分布产生较大影响。现场及卫星观测表明，台风等极端天气影响浮游植物时空分布的4个主要影响因子为：海水的垂直混合、流域的降水增加、河流淡水及营养盐的大量输入及风场变化。但是，尚未对这些因子如何影响叶绿素分布进行过清晰阐释。研究构建了粤港澳大湾区陆海气集成模拟系统，一方面通过控制实验来对模式敏感性进行测试，另一方面以台风"天鸽"过境珠江口为例，来对台风过境期间海表叶绿素时空变化机理进行探究。结果表明，陆地模式径流分辨率的提高对模拟结果有很大影响，台风"天鸽"过境时，向岸的风抑制了珠江冲淡水排出，促进外海水涌入，提高海表盐度，降低叶绿素浓度。而台风所致的陆地降水在过境后四天（8月27日）通过径流输出至河口，此次剧烈的增水向珠江口输入大量的营养盐，促进浮游植物持续生长，使叶绿素浓度在台风过境两周（9月1日）后仍在增加。研究所构建的集成模拟系统可适于其它受河口面源污染影响的海湾及近海系统。为未来实现赤潮、缺氧等生态灾害预报提供可能性。 Abstract:Over the last 10 years, progressive development of the Guangdong-Hong Kong-Macau Greater Bay area in our country has caused rapid population growth within the Pearl River watershed. In the meantime, demand for food and energy has increased dramatically. These result an expansion of agriculture land in the upper reaches of the Pearl River catchment and an increase in the rate of urbanization in the downstream area. As a result, groundwater and surface runoff entering the Pearl River up-stream now contains higher levels of fertilizer than previous, whilst in urban areas sewage and effluent production has increased considerably. A side effect of these changes is that the amount of pollution entering the estuary in a non-point source manner has increased, leading to widespread eutrophication there. Besides, global warming increased typhoons from South China Sea and Western Pacific making landfall near the Pearl River estuary, resulting in the enhanced redistribution of anthropogenic nutrients into the oceanic water. Both in-situ and satellite have been used to investigate the impact these typhoons have on Pearl River phytoplankton dynamics. They identified four factors impacting the redistribution of phytoplankton include the vertical mixing of oceanic water, high precipitation in the watershed, the large freshwater discharge and nutrients loading, and the variation of wind fields. To date, there have been no studies address how these four factors work together to influence the distribution of chlorophyll in the Pearl River estuary. In this research, we built a land-ocean-atmosphere modeling system for the China Great Bay Area. We conducted numerous controlled experiments to examine the sensitivity of the system to multiple forcing conditions. We also investigated the effect of each controlling mechanism has on phytoplankton dynamics, using the incidence of Hurricane Hato's arrival in the area as an opportunity to collect first-hand, real-time observations. Our results show that the temporal resolution of river runoff data used in the modeling system has a profound impact on the simulation results. Using daily river runoff data from land surface models produced very different results from those using monthly data. As Hato passed by, strong onshore winds inhibited the spread of freshwater from the Pearl River, increasing the influx of seawater which in turn increased sea surface salinity and reduced the concentration of surface chlorophyll. Four days (August 27th) after Hato had passed by, water that had fallen on land as precipitation during the typhoon entered the Estuary as runoff. The dramatic increase of river discharge coincident with this event enhanced the level of nutrient loading and caused dramatic phytoplankton growth. Chlorophyll levels were still increasing two weeks after the typhoon Hato passing by (September 1st). The land-ocean-atmosphere modeling system built for the Pearl River Estuary in our research could be adapt to other estuarine and coastal regions and would ultimately help predict algal blooms, hypoxia within the aquatic environment, and other ecological hazards in the future. 参考文献 相似文献 引证文献

Nutrient Loading, Temperature and Heat Wave Effects on Nutrients, Oxygen and Metabolism in Shallow Lake Mesocosms Pre-Adapted for 11 Years

Global changes (e.g., warming and population growth) affect nutrient loadings and temperatures, but global warming also results in more frequent extreme events, such as heat waves. Using data from the world’s longest-running shallow lake experimental mesocosm facility, we studied the effects of different levels of nutrient loadings combined with varying temperatures, which also included a simulated 1-month summer heat wave (HW), on nutrient and oxygen concentrations, gross ecosystem primary production (GPP), ecosystem respiration (ER), net ecosystem production (NEP) and bacterioplankton production (BACPR). The mesocosms had two nutrient levels (high (HN) and low (LN)) combined with three different temperatures according to the IPCC 2007 warming scenarios (unheated, A2 and A2 + 50%) that were applied for 11 years prior to the present experiment. The simulated HW consisted of 5 °C extra temperature increases only in the A2 and A2 + 50% treatments applied from 1 July to 1 August 2014. Linear mixed effect modeling revealed a strong effect of nutrient treatment on the concentration of chlorophyll a (Chl-a), on various forms of phosphorus and nitrogen as well as on oxygen concentration and oxygen percentage (24 h means). Applying the full dataset, we also found a significant positive effect of nutrient loading on GPP, ER, NEP and BACPR, and of temperature on ER and BACPR. The HW had a significant positive effect on GPP and ER. When dividing the data into LN and HN, temperature also had a significant positive effect on Chl-a in LN and on orthophosphate in HN. Linear mixed models revealed differential effects of nutrients, Chl-a and macrophyte abundance (PVI) on the metabolism variables, with PVI being particularly important in the LN mesocosms. All metabolism variables also responded strongly to a cooling-low irradiance event in the middle of the HW, resulting in a severe drop in oxygen concentrations, not least in the HN heated mesocosms. Our results demonstrate strong effects of nutrients as well as an overall rapid response in oxygen metabolism and BACPR to changes in temperature, including HWs, making them sensitive ecosystem indicators of climate warming.

Biogeographic patterns of abundant and rare bacterial and microeukaryotic subcommunities in connected freshwater lake zones subjected to different levels of nutrient loading.

To reveal whether the patterns of abundant and rare subcommunity composition of both bacteria and microeukaryotes vary between connected regions with different levels of nutrient loading in freshwater lakes. We investigated the abundant and rare subcommunity composition of both bacteria and microeukaryotes in two connected zones (Meiliang Bay (MLB) and Xukou Bay (XKB)) of a large shallow freshwater Lake Taihu via the high-throughput sequencing of bacterial 16S rRNA and microeukaryotic 18S rRNA genes. Even though these two lake zones are connected and share a species bank, they diverge in community composition. Significantly higher alpha diversity was observed for the abundant bacterial subcommunity in the MLB. However, no significant difference in alpha diversity between the rare bacterial subcommunities, as well as both rare and abundant microeukaryotic subcommunities were observed between MLB and XKB. It is demonstrated that both environmental factors and geographic distance play central roles in controlling the rare and abundant microbial subcommunities in the two connected lake zones. The abundant subcommunity composition of bacteria and microeukaryotes vary between connected regions with different levels of nutrient loading. Dispersal limitation plays a vital role in shaping microbial communities even in connected zones of freshwater lakes. Leading to a comprehensive understanding of the characteristics of microbial community in connected lake regions with different levels of nutrient loading.

Effects of nutrient levels and light intensity on aquatic macrophyte (Myriophyllum aquaticum) grown in floating-bed platform

In recent years, Myriophyllum aquaticum grown on floating beds has been widely applied to the treatment of the malodorous black river water with high nutrient concentrations. This is due to the ability of the effective removal of nutrients by this plant. However, environmental factors such as light and nutrients influence plant growth, which may negatively affect the results of ecological restoration. To test the effects of nutrient levels and light intensity on the growth of M. aquaticum, a 21-day outdoor two-factorial experiment (three levels of nutrient loadings and three light intensities) was conducted from September 3 to September 30, 2016. Results showed that the addition of nutrients had no significant effects on the growth of M. aquaticum. Reduced light intensity led to a decrease in Chlorophyll a concentration in the water column and the growth of M. aquaticum. All growth indicators in the full sunlight treatment were significantly higher than in the shading treatments. The lowest growth was found in the 60% shading treatment. The mean plant height and the number of roots responded significantly to the interaction between light intensity and nutrient level. This study reveals that M. aquaticum planted on the floating beds could grow normally in eutrophic waters and can be used as a restoration technique to treat eutrophic water. However, shading (e.g., riparian trees) may be a major limiting environmental factor to M. aquaticum.

Ambient nutrient availability drives the outcome of an interaction between a sponge (Halichondria melanadocia) and seagrass (Thalassia testudinum)

Nutrient loading is a common human impact in coastal habitats which is driving significant losses of seagrass habitat worldwide. This is concerning, as seagrass beds provide numerous ecosystem services. Although nutrient loading can result in eutrophication and mass mortality of seagrasses, it more often results in subtle alterations of abiotic conditions which, by themselves, are insufficient to drive widespread seagrass mortality. Here we used a month-long field-based experiment to test the influence of anthropogenic nutrient loading on the outcome of an interaction between an epizootic sponge (Halichondria melanadocia) and a seagrass (Thalassia testudinum). Using a factorial design we manipulated ambient nutrient concentrations by adding fertilizer to plots with and without a sponge in a relatively pristine seagrass bed on Abaco Island, The Bahamas. We measured seagrass growth, below- and above-ground biomass, and the change in shoot density. We found that low levels of nutrient loading (i.e., insufficient to cause increased algal growth and decreased oxygen levels associated with eutrophication) resulted in a non-significant increase in seagrass growth, biomass, and shoot density when a sponge was absent. When a sponge was present, the same level of fertilization resulted in a significant reduction in the response variables. Our results show that anthropogenic nutrient loading can shift the sponge-seagrass interaction away from commensalism toward an interaction that has negative consequences for the seagrass. Additionally, a shift in the outcome of this context-dependent interaction can alter the impact of nutrient loading on seagrass productivity. Our study provides an example of how simple classifications of interspecific interactions (e.g., commensalism) often mask underlying variability. Characterizing the mechanisms driving the variability will allow us to understand when and how the outcome of an interaction will change. Ultimately, this will allow us to better predict how human activities will indirectly impact various ecosystem functions.

Eutrophic water or fertile sediment: which is more important for the growth of invasive aquatic macrophyte Myriophyllum aquaticum?

Invasive macrophyte Myriophyllum aquaticum is capable of assimilating nutrients from both the sediments and the water column. However, which is the major source of nutrients supporting M. aquaticum growth under various nutrient regimes has not been well documented. Here we conducted a two-factorial outdoor experiment (three levels of nutrient loading and two types of sediments) from 23 May to 27 June 2016, to assess M. aquaticum responses to different nutrient levels in the water column and the sediment. Results showed that concentrations of total nitrogen, total phosphorus and chlorophyll a in the water column increased in the treatment groups, but decreased slightly in the control group (nutrient-poor sediment and no nutrient addition). Sediment type had a significant effect on the growth M. aquaticum, while there were no significant effects of nutrient loading levels and the interactions between the two factors. Mean relative growth rate, mean plant height, mean stem diameter, the number of lateral branches and roots in the nutrient-rich sediment treatments were 1.6, 1.2, 1.6, 3.2 and 5.9 folds greater than in the nutrient-poor sediment treatments, respectively. These results suggest that nutrient-rich sediment has a positive effect on the growth of M. aquaticum. This study reveals possible expansion mechanism of the exotic M. aquaticum in China, and may provide valuable information on the ecological restoration of eutrophic freshwaters.

Macrofaunal Functional Diversity Provides Resilience to Nutrient Enrichment in Coastal Sediments

The degradation of ecosystems is often associated with losses of large organisms and the concomitant losses of the ecological functions they mediate. Conversely, the resilience of ecosystems to stress is strongly influenced by faunal communities and their impacts on processes. Denitrification in coastal sediments is a process that may provide ecosystem resilience to eutrophication by removing excess bioavailable nitrogen. Here, we conducted a large-scale field experiment to test the effect of macrofaunal community composition on denitrification in response to two levels of nutrient enrichment at 28 sites across a biologically heterogeneous sandflat. After 7 weeks of enrichment, we measured denitrification enzyme activity (DEA) along with benthic macrofaunal community composition and environmental variables. We normalised treatment site specific DEA values by those in ambient sediments (DEACN) to reveal the underlying response across the heterogeneous landscape. Nutrient enrichment caused reductions in DEACN as well as functional changes in the community; these were both more pronounced under the highest level of nutrient loading (on average DEACN was reduced by 34%). The degree of suppression of DEACN following moderate nitrogen loading was mitigated by a key bioturbating species, but following high nitrogen loading (which reduced the key species density) the abundance and diversity of other nutrient processing species were the most important factors alleviating negative effects. This study provides a prime example of the context-dependent role of biodiversity in maintaining ecosystem functioning, underlining that different elements of biodiversity can become important as stress levels increase. Our results emphasise that management and conservation strategies require a real-world understanding of the community attributes that facilitate nutrient processing and maintain resilience in coastal ecosystems.

Response of Vallisneria natans to Increasing Nitrogen Loading Depends on Sediment Nutrient Characteristics

High nitrogen (N) loading may contribute to recession of submerged macrophytes in shallow lakes; yet, its influences vary depending on environmental conditions. In August 2013, we conducted a 28-day factorial-designed field mesocosm experiment in Lake Taihu at the Taihu Laboratory for Lake Ecosystem Research (TLLER) to examine the effects of high N loading on the growth of Vallisneria natans in systems with contrasting sediment types. We ran the experiments with two levels of nutrient loading—present-day external nutrient loading (average P: 5 μg·L−1·day−1, N: 130 μg·L−1·day−1) and P: 5 μg·L−1·day−1, and with three times higher N loading (N: 390 μg·L−1·day−1) and used sediment with two contrasting nutrient levels. V. natans growth decreased significantly with increasing N loading, the effect being dependent, however, on the nutrient status of the sediment. In low nutrient sediment, relative growth rates, leaf biomass and root biomass decreased by 11.9%, 18.2% and 23.3%, respectively, at high rather than low N loading, while the decline was larger (44.0%, 32.7% and 41.8%, respectively) when using high nutrient sediment. The larger effect in the nutrient-rich sediment may reflect an observed higher shading of phytoplankton and excess nutrient accumulation in plant tissue, though potential toxic effects of the high-nutrient sediment may also have contributed. Our study confirms the occurrence of a negative effect of increasing N loading on submerged plant growth in shallow nutrient-enriched lakes and further shows that this effect is augmented when the plants grow in nutrient-rich sediment. External N control may, therefore, help to protect or restore submerged macrophytes, especially when the sediment is enriched with nutrients and organic matter.

Statistical Dimensioning of Nutrient Loading Reduction: LLR Assessment Tool for Lake Managers.

Implementation of the EU Water Framework Directive (WFD) has set a great challenge on river basin management planning. Assessing the water quality of lakes and coastal waters as well as setting the accepted nutrient loading levels requires appropriate decision supporting tools and models. Uncertainty that is inevitably related to the assessment results and rises from several sources calls for more precise quantification and consideration. In this study, we present a modeling tool, called lake load response (LLR), which can be used for statistical dimensioning of the nutrient loading reduction. LLR calculates the reduction that is needed to achieve good ecological status in a lake in terms of total nutrients and chlorophyll a (chl-a) concentration. We show that by combining an empirical nutrient retention model with a hierarchical chl-a model, the national lake monitoring data can be used more efficiently for predictions to a single lake. To estimate the uncertainties, we separate the residual variability and the parameter uncertainty of the modeling results with the probabilistic Bayesian modeling framework. LLR has been developed to answer the urgent need for fast and simple assessment methods, especially when implementing WFD at such an extensive scale as in Finland. With a case study for an eutrophic Finnish lake, we demonstrate how the model can be utilized to set the target loadings and to see how the uncertainties are quantified and how they are accumulating within the modeling chain.

Functional traits of herbivores and food chain efficiency in a simple aquatic community model

Large-bodied zooplankton play important roles in the flow of nutrients and energy along the food chain in freshwater ecosystems. However, the importance of functional traits of zooplankton in trophic flow is not well understood. We used the minimal food-chain model, which includes producers (algae), first-order consumers (grazer zooplankton), and secondary consumers (fish), to reveal which functional traits of phyto- or zooplankton affect one of the most important ecosystem processes for functioning, the trophic transfer efficiency (TTE) across three trophic levels. Numerical simulations indicated that, regardless of the level of nutrient loading and the fish density, the conversion coefficient of grazer zooplankton was one of the most important and general factors for determining TTE. The antipredator defense and maximum grazing rate of zooplankton greatly affected TTE, but the effect depended on the level of nutrient loading and the fish density. For an ecosystem that had high nutrient loading and high fish density and in which the zooplankton density was regulated by the top-down effect, antipredator defense and maximum grazing enhanced TTE. Opposite relationships were observed for other states of the ecosystem. In general, those functional traits of the first-order consumers that affected vertical ecological interaction were important for trophic flow in the ecosystem, whereas the sensitivity of trophic flow to these traits depended largely on whether the zooplankton were regulated by the bottom-up or the top-down effect.

Distance-decay patterns of nutrient loading at watershed scale: Regression modeling with a special spatial aggregation strategy

The effect of distance-decay on nutrient flux usually plays an important role in nutrient retention from non-point sources to surface waters. However, the distance-decay effect has been inappropriately neglected in many studies that adopted regression modeling method to quantify the relationship between watershed landscape and in-stream nutrient loading level. The goal of this study was to develop non-linear regression models that better quantify the role of distance on non-point source nutrient loads in rivers by using simulation results of a spatially-explicit model applied to the watersheds in the Upper Chattahoochee River Basin, Georgia. Because a detailed spatially-explicit modeling approach typically simulates the nutrient flux across the entire watershed, it provides opportunities to examine the nutrient decay patterns at watershed scale. The simulation results confirmed that regarding the effect of flow distance on nutrient loading, the exponential decay setting in the spatially-explicit model performs well. Other heterogeneous factors including slope and soil conditions do affect the decay results but not strongly enough to change the general exponential patterns. The nutrient contribution from areas that were greater than 300 meters to the river network was negligible. It was also found that the decay rate for urban lands is lower than that for other land-covers. Based on these findings, a spatial aggregation strategy in the lateral dimension of the river network was adopted and eight non-linear regression models which explicitly addressed the effects of distance-decay were designed to estimate the nutrient annual average loads. The model validation results showed that three of them can well estimate the nutrient loads. This study shows the usage of stream-lateral-dimension aggregation strategy in addressing nutrient distance-decay patterns and developing simple regression models of nutrient loading. This study also illustrates the advantages of using comprehensive spatially-explicit models to design and configure simpler regression models for improving our understanding of nutrient export from land to water.

The Growth of Estuarine Resources (Zostera marina, Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians, Cyprinodon variegatus) in Response to Nutrient Loading and Enhanced Suspension Feeding by Adult Shellfish

While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspension-feeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.

Nutrient loading of Norway spruce seedlings hastens bud burst and enhances root growth after outplanting

We studied the effects of late season nutrient loading (NLOAD) on the timing of bud burst, growth and changes in nitrogen (N) concentrations in the first growing season after seedlings were outplanted. Two-year-old Norway spruce (Picea abies (L.) Karst.) seedlings with three foliar nitrogen concentration levels (NLOAD levels 11.3, 22.5 and 27.5 g N kg for L, M- and H-seedlings, respectively) were examined in the following three experiments: root growth capacity test (RGC), rooting experiment in the field and soil fertility experiment (‘rich’ or ‘poor’ soil) in the field. Bud burst in RGC was monitored daily and foliar N concentration (field experiments), height and root growth (rooting experiment) at monthly intervals. With respect to the RGC test, no differences in root growth were observed among the three NLOAD levels, but buds of H-seedlings burst 2–6 days earlier than others. In the rooting experiment, nutrient loading increased height and root growth but did not affect the timing of height growth. In the soil fertility experiment, foliar N of H- and M-seedlings decreased rapidly, but the decline was slower in rich soil. Current-year needles had more N in seedlings growing in rich soil and the N concentration declined until height growth ceased whereafter it increased until autumn. Improved growth from nutrient loading seems to last only for the first season after planting and the greatest benefits are enjoyed by seedlings planted in poor soils.-1

Effect of water aeration and nutrient load level on biomass yield, N uptake and protein content of the seaweed Ulva lactuca cultured in seawater tanks

The effects of 16 different combinations of nutrient load and agitation on yield, nutrient uptake and proximate chemical composition of the seaweed Ulva lactuca cultured in tanks were evaluated. Intensive fishpond outflow passed through seaweed tanks at four nutrient loading levels and four water agitation combinations of water exchange, bottom aeration and frequently changing water levels (an accelerated tide regime). Specific results from these outdoor experiments were examined further under controlled conditions in laboratory experiments. Agitation treatments affected the performance of U. lactuca only under TAN (\({\text{NH}}_{3} + {\text{NH}}^{ + }_{4} \)) load levels below 4 g N m−2 day−1; biofiltration of TAN was the parameter most affected. Biomass yields at each of the four nutrient loading levels were not significantly different between the agitation treatments. Protein content increased significantly with increasing nutrient loading. The agitation treatments had a slight effect on seaweed protein content only at the lowest nutrient loading levels. There were no significant differences in dissolved oxygen concentration, pH, and temperature among the agitation treatments at all nutrient loading levels. Under laboratory conditions, growth rates, protein content, and photosynthetic and biomass yield of the seaweed were affected by water velocity under low nutrient concentrations. It is concluded that the effect of air agitation under the conditions of these experiments was not directly related to photosynthesis, excess dissolved oxygen, or carbon limitation, but to the diffusion of macro nutrients from the water to the seaweed. Therefore, once nutrient concentrations are high enough (above about 4 μM of TAN with the other nutrients in their corresponding proportions), aeration per se is not essential for effective growth and biofiltration by seaweeds.

BARNEGAT BAY–LITTLE EGG HARBOR ESTUARY: CASE STUDY OF A HIGHLY EUTROPHIC COASTAL BAY SYSTEM

The Barnegat Bay–Little Egg Harbor Estuary is classified here as a highly eutrophic estuary based on application of the National Oceanic and Atmospheric Administration's National Estuarine Eutrophication Assessment model. Because it is shallow, poorly flushed, and bordered by highly developed watershed areas, the estuary is particularly susceptible to the effects of nutrient loading. Most of this load (∼50%) is from surface water inflow, but substantial fractions also originate from atmospheric deposition (∼39%), and direct groundwater discharges (∼11%). No point source inputs of nutrients exist in the Barnegat Bay watershed. Since 1980, all treated wastewater from the Ocean County Utilities Authority's regional wastewater treatment system has been discharged 1.6 km offshore in the Atlantic Ocean. Eutrophy causes problems in this system, including excessive micro‐ and macroalgal growth, harmful algal blooms, altered benthic invertebrate communities, impacted harvestable fisheries, and loss of essential habitat (i.e., seagrass and shellfish beds). Similar problems are evident in other shallow lagoonal estuaries of the Mid‐Atlantic and South Atlantic regions. To effectively address nutrient enrichment problems in the Barnegat Bay–Little Egg Harbor Estuary, it is important to determine the nutrient loading levels that produce observable impacts in the system. It is also vital to continually monitor and assess priority indicators of water quality change and estuarine health. In addition, the application of a new generation of innovative models using web‐based tools (e.g., NLOAD) will enable researchers and decision‐makers to more successfully manage nutrient loads from the watershed. Finally, the implementation of storm water retrofit projects should have beneficial effects on the system.

Salinity Induced Regime Shift in Shallow Brackish Lagoons

In brackish lagoons, Daphnia is replaced by calanoid copepods (Eurytemora affinis, Acartia spp.) and rotifers when a certain threshold (depending on, for instance, fish density) is reached. We hypothesize that loss of Daphnia induces a regime shift from clear to turbid at high nutrient concentrations. We conducted a factorial designed enclosure experiment with contrasting salinities (0–16‰), low fish predation (one three-spined stickleback, Gasterosteus aculeatus, m−2) and three levels of nutrient loading in a shallow brackish lagoon. A change point analysis suggests a strong regime shift from a clear to a turbid state at 6–8‰ salinity at low and high loading, but not for the control. From the low to the high salt regime, chlorophyll a (Chla), Chla:total phosphorus (TP) and Chla:total nitrogen (TN) ratios shifted highly significantly for all nutrient treatments, and the bacterioplankton production followed the changes in Chla. These changes occurred parallel with a shift from cladoceran and cyclopoid copepod to rotifer dominance. Monitoring data from 60 Danish brackish lagoons show increasing Chla with increasing TP and TN as well as interactive effects of TN and salinity, peaking at intermediate salinity. A relatively weak effect of salinity at low nutrient concentrations and the stronger effect at intermediate high salinity are in accordance with the experimental results. However, these data suggest a lower salinity threshold than in the experiment, which may be explained by a higher fish density. Our results have implications for the management of coastal lagoons both at present and in a future (predicted) warmer climate: (1) improved water quality can be obtained by reducing the nutrient loading or enhancing the freshwater input to a level triggering a shift to Daphnia dominance (typically <2‰), (2) fish manipulation is probably not a useful tool for brackish lagoons, unless the salinity is below the threshold for a potential shift to a clear Daphnia dominated state, and (3) more abrupt changes will expectedly occur in low-saline coastal lagoons at increasing salinity during summer in a future warmer climate.

Seasonal variability in response of estuarine phytoplankton communities to stress: Linkages between toxic trace elements and nutrient enrichment

We examined individual and interactive effects of two stressors—nutrients (nitrogen [N] and phosphorus [P]) and trace elements (a mix of arsenic [As], copper [Cu], and cadmium [Cd], and in a second experiment also zinc [Zn] and nickel [Ni])—on phytoplankton of the mesohaline Patuxent River, a tributary of Chesapeake Bay. Experiments were conducted in twenty 1-m3 mesocosms. Four mesocosm runs used two levels of nutrient loadings (0.7–1.0 × ambient N loading and enriched to 1.3–1.6 × ambient N loading) crossed with two levels of trace elements (ambient and enriched approximately 2–5 × higher than ambient concentrations) crossed with five progressive levels of ecosystem complexity. To examine seasonal patterns of responses to stressors, data from these experiments were combined with results of a similar experiment conducted during 1996 (Breitburg et al. 1999a). A second mesocosm experiment examined effects of individual and mixed trace elements, both alone and in combination with nutrients, to further examine which nutrient-trace element interactions were important. Nutrients consistently increased phytoplankton productivity and biomass. Most of the increased biomass was created by large centric diatoms, which increased the mean cell size of the phytoplankton community. Trace element additions decreased phytoplankton productivity and biomass, as well as the contribution of large centric diatoms to phytoplankton biomass. When both trace elements and nutrients were added, trace elements reduced nutrient stimulation. Although the magnitude of the response to nutrient additions tended to be somewhat greater in spring, the seasonal patterns of trace element effects, and nutrient-trace element interactions were far more striking with significant responses restricted to spring mesocosm runs. The second experiment indicated that both As and Cu were more inhibitory to phytoplankton in spring than in summer, but As was more inhibitory in the low nutrient treatments and Cu was more inhibitory in the nutrient enrichment treatments. The potential for strong seasonal patterns and high temporal variability in stressor effects and multiple stressor interactiosn will require close attention in the design and interpretation of management-relevant research and monitoring and may indicate the need for seasonally varying management strategies.