The impact of extreme weather events on the seagrass Zostera noltii and related Hydrobia ulvae population

Coastal areas are typically subjected to a range of stressors, but they now face the additional stressor of climate change, manifested in part by an increased intensity and frequency of extreme weather events. Thus, the Mondego estuary (Portugal) has experienced organic enrichment (eutrophication) issues and these are potentially exacerbated by extreme weather events (floods, droughts and heat waves). In this paper, we explore the impact of interactions of these different stressors on the ecology of the system, specifically on the two key components, the seagrass Zostera noltii and the mud snail Hydrobia ulvae. Extreme events affected different components of the estuarine ecosystem (primary producers and macrofauna) differently. Whilst the floods directly impacted on H. ulvae, by wiping out part of its population, they did not directly affect the biomass of Z. noltii. In contrast, drought events, through their effects on salinity, directly impacted the biomass of Zostera, which had knock-on effects on the dynamics of H. ulvae. We conclude that over the period when the estuary experienced eutrophication, extreme weather events contributed to the overall degradation of the estuary, while during the recovery phase following the introduction of a management programme, those extreme weather episodes delayed the recovery process significantly.     

Empirical relationship between eelgrass extent and predicted watershed-derived nitrogen loading for shallow New England estuaries

Seagrasses provide important ecological services that directly or indirectly benefit human well-being and the environment. Excess nitrogen inputs are a major cause of eelgrass loss in the marine environment. Here we describe the results of a study aimed at quantifying the extent of eelgrass as a function of predicted watershed-derived nitrogen loading for small-to-medium-sized shallow estuaries in New England. Findings confirm that reduced extent of eelgrass corresponds to increased loading of nitrogen to this class of estuary. At lower levels of nitrogen loading (≤50 Kg ha⁻¹ yr⁻¹), eelgrass extent is variable and is likely controlled by other ecosystem factors unrelated to water quality. At higher loading rates, eelgrass coverage decreases markedly, with essentially no eelgrass at loading levels ≥100 Kg ha⁻¹ yr⁻¹.     

南海大鹏湾海洋褐胞藻赤潮及其成因

于１９９１年３月２０－２１日在大鹏湾首次发生海洋褐胞藻赤潮。对此分析了赤潮发生前后海洋环境因素的变化及其与赤潮的关系；以风速，气压，盐度，温度，磷酸盐，硝酸盐，铵盐，铁离子等８项指标为基础，改变参数组合，对采自１９９１年３－５月特定站位（Ｓｏ）的各样方进行聚类分析和比较。结果表明，在盐度为３１－３２和水温为２０℃的适宜条件下，铁离子和风速是形成本次赤潮的主要环境要素；大量营养盐尤其是硝酸盐度的增加     

海洋尖尾藻的摄食

海洋尖尾藻(Oxyrrhis marina)是世界广布的一种赤潮原因甲藻。以亚心形扁藻(Platymonas subcordiformis)和米氏凯伦藻(Karenia mikimotoi)等6种海洋微藻为饵料对海洋尖尾藻的摄食进行了显微观察。结果表明:海洋尖尾藻的摄食方式为滤食,整个摄食过程分为水流形成和胞吞作用两个阶段;水流形成依靠海洋尖尾藻整个细胞围绕饵料藻旋转实现,胞吞器官为纵沟;每个海洋尖尾藻细胞内可同时容纳饵料藻一至数个。此外,分属于不同门类的供试藻均能被海洋尖尾藻摄食。     

Seagrass tolerance to herbivory under increased ocean temperatures

Climate change is acknowledged as a major threat to marine ecosystems, but the effect of temperature on species interactions remains poorly understood. We quantified the effects of long-term warming on plant-herbivore interactions of a dominant seagrass, Zostera muelleri. Growth, herbivory and tolerance to damage were compared between a meadow warmed by the thermal plume from a power station for 30 years (2–3 °C above background temperatures) and three control locations. Leaf growth rates and tissue loss were spatially variable but unrelated to temperature regimes. Natural herbivory was generally low. Simulated herbivory experiments showed that the tolerance of Z. muelleri to defoliation did not differ between warm and unimpacted meadows, with damaged and undamaged plants maintaining similar growth rates irrespective of temperature. These results suggest that the ability of temperate Z. muelleri to tolerate herbivory is not strongly influenced by warming, and this species may be relatively resilient to future environmental change.     

Dependence of eelgrass (Zostera marina) light requirements on sediment organic matter in Massachusetts coastal bays: Implications for remediation and restoration

Using a calibrated bio-optical model we determined that the optical water quality conditions in several nitrogen-impaired embayments and in one unimpaired system were within the range of values known to support eelgrass growth. We also used the model to identify a range of light requirements for eelgrass (Zostera marina). Higher eelgrass light requirements, expressed as a percentage of surface-incident irradiance, corresponded with higher sediment organic matter content. These results corroborated findings by previous studies which indicate a generalized relationship: seagrasses growing in turbid conditions with poorer water and sediment quality have higher light requirements than those growing in less degraded conditions. The mechanistic reason for the variation in light requirements is still not completely explained and cannot be attributed to a single independent variable. Varying light requirement have important implications for eelgrass protection and should be considered when setting restoration targets for eelgrass in water quality and nitrogen remediation programs.     

The impact of extreme flooding events and anthropogenic stressors on the macrobenthic communities’ dynamics

Marine and coastal environments are among the most ecologically and socio-economically important habitats on Earth. However, climate change associated with a variety of anthropogenic stressors (e.g. eutrophication) may interact to produce combined impacts on biodiversity and ecosystem functioning, which in turn will have profound implications for marine ecosystems and the economic and social systems that depend upon them.     

The response of primary producer assemblages to mitigation measures to reduce eutrophication in a temperate estuary

The Mondego estuary is a well-described system located on the North Atlantic Ocean, where cultural eutrophication progressed over the last decades of the 20th century. Consequently, and due to a large productivity of Ulva spp., Zostera noltii meadows were severely reduced with a concomitant decrease in environmental quality. In 1998, experimental mitigation measures were implemented, via changes in hydrology to increase circulation and diversion of nutrient-rich freshwater inflow, to reverse the process in the most affected area of the estuary – its South arm.The objective of this study was to assess the differences in response of primary producer assemblages to the implemented measures to reduce eutrophication.Results show that the mean concentrations of DIN suffered a notorious decrease due to a significant reduction in the ammonium concentration in the water column, while DIP increased significantly. Primary producer assemblages showed different responses to these changes: phytoplankton, measured as concentration of chlorophyll a, did not show any significant changes; green macroalgae, mostly Ulva spp., suffered a large reduction in biomass, whereas Gracilaria gracilis and the macrophyte Zostera noltii biomasses increased greatly. Results show that phytoplankton biomass has remained constant and suggest that the reduction in ammonium could have been responsible for the changes in the green macroalgal biomass. Light was the most likely factor in the response of seagrass whereas red macroalgal reaction seemed to be dependent on both light and ammonium.