Estuaries and Coasts最新文献

Conservation efforts have raised awareness about the impact of small-scale fisheries on the distribution of seagrass plants. The patterns of recovery of the seagrass Zostera noltei and of the commercial bivalves Cerastoderma edule, Ruditapes decussatus and Ruditapes philippinarum after shellfish harvesting were studied in a field experiment in a shellfish bed in NW Spain. Sample plots were subjected to a single disturbance in two types of shellfish harvesting treatments in three zones characterized by different harvesting frequency and seagrass density. The photosynthetic efficiency (F_v/F_m), shoot density, leaf length and carbohydrate content of Z. noltei were monitored every three months during one year, and the C and N content of leaves and biomass of plants were measured one year after the disturbance. The abundance of adults, juveniles and recruits and the condition index of adult bivalves were quantified after the experimental harvesting. Shoot density and biomass of Z. noltei remained low during the ten months after the disturbance but recovered to control values after one year. Carbohydrate contents of apical rhizomes were lower in disturbed (treated) plots, whereas no effect was observed on F_v/F_m. Denser and more complex seagrass patches recovered faster. The abundance of adult bivalves below commercial size was lower in the disturbed plots, while the abundance of adults of commercial size, juveniles and recruits did not vary, indicating that abundance and condition were not hampered by harvesting pressure. The findings also suggest that Z. noltei meadows can recover within one year of the impact of shellfish harvesting if the havesting areas are rotated and dense patches are preserved.

As a consequence of anthropogenic activities and climate change, accelerated terrestrial sediment runoff is causing the gradual mudification of soft sediment estuarine habitats worldwide. Increased sediment mud content (< 63 µm) has been recognised to alter seagrass morphology and cause declines in primary production in unvegetated habitats. However, the effect of increased mud content on primary production in seagrass meadows remains largely unknown. To address this, primary production in intertidal seagrass meadows (Zostera muelleri) and adjacent unvegetated habitats was measured in situ using benthic incubation chambers across an existing sedimentary gradient (nine sites spanning 5–33% mud content). An additional two unvegetated mudflat sites (39–49% mud content) were also sampled to expand the gradient. Seagrass net (NPP) and gross primary production (GPP) was greater than in the adjacent unvegetated habitat and did not vary with mud content, even after standardising GPP by photosynthesising biomass (i.e. photosynthetic efficiency). In contrast, in the adjacent unvegetated habitat, photosynthetic efficiency declined with increasing mud content. Inclusion of the additional mudflat sites negatively impacted NPP, GPP, and photosynthetic efficiency in the unvegetated habitat. Thus, while primary production in seagrass meadows may have some resilience to future increases in mud content (up to ~33%), further degradation and loss of seagrass habitats could result in the expansion of unvegetated habitats and ultimately lead to production losses, likely to be most acute in areas with high mud content (≥ 39%).

Enhalus acoroides (L.f.) Royle is a large and important foundation seagrass species in the tropical Indo-Pacific. The northern marginal populations in China have been declining over the last decades. The reproductive phenology and fruit set of this dioecious seagrass were investigated from June 2017 to February 2018 in two areas in Li’an lagoon, Hainan Island, China. We found that E. acoroides flowered year-round. However, the flowering intensity varied among seasons and was highest in the summer and lowest in the winter. The shoot sex ratio was male-biased, and the pollen-ovule ratio was approximately 10⁴: 1, based on the sex ratio and the number of male florets per inflorescence. Female and male plants flowered at the same time in this population. Despite the high reproductive investment in terms of flower formation observed throughout the year, the abundance of the mature fruits was low, possibly due to physical disturbance and cover by epiphyte and algae. This study provides insight into the reproductive ecology of E. acoroides, which will be useful for the future conservation of this threatened seagrass.

Impermeable infrastructure such as traffic causeways can reduce the natural hydrodynamic flushing of an estuary, resulting in reduced water quality and increased incidence of harmful algal blooms (HABs). A series of cuts through the three causeways spanning Old Tampa Bay, FL, (OTB) are being considered to help restore the natural circulation of the region, but the number of possible location combinations is computationally challenging to fully assess. A prototype genetic algorithm (GA) was developed to identify the optimal configuration of these cuts through one of the bridge sections that maximizes flushing as represented in a numerical ocean circulation model of OTB. Flushing was measured by integrating the trajectories of over 21,000 passive Lagrangian “particles” using the model velocity fields. The rate of loss of particles initialized near Feather Sound (a region subject to frequent HABs) was used to quantify the “fitness” over which the configurations were optimized. The highest-scoring solution produced a 42% increase in net flushing compared to a no-change baseline. Six independently initialized applications of the GA were conducted. All converged to the same solution within no more than 7 generations. The small population size of the prototype allowed testing of the complete solution space, and verification the found solution was optimal. Elitism (preservation of the highest-ranking solution) was required for convergence. The GA also identified configurations that had similar, but slightly slower, flushing rates. These results will help area managers prioritize or rank combinations of causeway modifications to improve overall water quality conditions in Tampa Bay.

Marsh lateral expansion and retreat are often attributed to sediment availability, but a causal link is difficult to establish. To shed light on this problem, we analyzed changes in salt marsh area along the ~ 200-km-long Georgia coast (USA) from the 1850s to 2010s in relation to total suspended sediment (TSS) and to proxies for river sediment input and local sediment resuspension. Marsh area is characterized by large gains and losses (up to 200 m²/m/yr), but relatively small net change (-50 to 50 m²/m/yr or -0.1 to 0.1%/yr). This has resulted in a general loss of marsh area, except close to the mouths of major rivers, where there is net gain. Net expansion rates decreased in the Savannah Estuary but increased in the Altamaha Estuary from the 1850s–1930s period to the 1930s–2010s period, which are consistent with observed decreases and likely increases in sediment discharge in the two estuaries, respectively. To explain the spatial patterns in the 1930s–2010s marsh area change, we estimated TSS from satellite measurements (2003 to 2020). Along the northern part of the Georgia coast, net marsh gain is positively correlated to the average TSS within the estuarine region. However, this correlation breaks down in more southern areas (Cumberland Sound). Coast-wide, there is a better correlation between TSS associated with new input from the rivers, estimated as the TSS difference between high-discharge (Jan–Mar) and low-discharge (Sept–Nov) months. To identify the effect of wave resuspension in the nearshore, we consider the TSS difference between high-wave, low-discharge (Sept–Nov) and low-wave, low-discharge periods (Jun–Aug). Wave resuspension is relatively uniform along the coast and does not explain spatial patterns of marsh area change. Sediment input from the nearshore is likely contributing to the estuarine sediment budget in Georgia, but it is not sufficient to prevent marsh lateral retreat. To identify the role of tidal resuspension and advection, we consider differences in TSS between low and high tide. This differential is relatively constant along most of the coast, but it is much lower in the southern part of the coast, suggesting a lower tidal action in this region. Sediment resuspended by tides is likely originating from internal recycling (i.e., erosion) within the estuary, and thus does not contribute to marsh lateral expansion. The proposed approach to partition TSS is a general demonstration and could be applied to other coastal regions.

Salt marshes have ecological and economic value, but shoreline development, the increasing rate of sea-level rise, and other human impacts have caused significant loss of salt marshes. As a result, restoration of these ecosystems is widespread. For restoration and management to be effective, it is imperative to improve our understanding of marsh-building plants that serve as the ecological foundation of these habitats. Given the observed differences in characteristics between populations of smooth cordgrass, Spartina alterniflora, restoration plantings may impact the biodiversity and resilience of restored ecosystems. Understanding differences in the structural and functional outcomes of active planting of restoration sites will enable the long-term success of restoration efforts to be improved. Natural and restored salt marshes in Long Island Sound were studied in 2021–2022 for S. alterniflora genetics, biomass, stem morphology, and faunal community composition. The average genotypic diversity of S. alterniflora was more than 4 times higher in restored than in natural marshes, and differentiation between each restored site and natural sites decreased with time. No difference was observed in live S. alterniflora belowground biomass; however, mean dead belowground biomass in natural marshes was more than 3 times greater than in restored marshes. Marsh platform invertebrates differed between the restored and natural sites, with natural marsh edge habitats having 9 times higher density of Geukensia demissa and 3 times as many crab burrows than in restored marshes, but there was no detected difference in species richness or abundance of nekton at high tide. With restoration practitioners seeking resilient, self-sustaining ecosystems, it is important to evaluate whether restored marsh characteristics are consistent with those goals and modify restoration planning accordingly to incorporate genetics, structure, and function.

Hypoxia is one of the predominant water quality issues affecting estuaries and coastal ecosystems, and its impact is often monitored using benthic macroinvertebrates. The M-AMBI (Multivariate AZTI Marine Biotic Index) is an index that meets the needs of small and large-scale monitoring as it is scalable. However, gaps remain as to the sensitivity of M-AMBI to hypoxia as few studies are available. Using Pensacola Bay in the northern Gulf of Mexico (USA) as a case study, we sought to evaluate the time scales over which benthic macrofauna respond to dissolved oxygen conditions from May through September 2017. Combined continuous DO monitoring and benthic sampling identified important differences in DO exposure on benthic habitat condition based on both the duration and frequency of low oxygen. We identified periods of 7 to 31 days as critical windows of exposure prior to a measurable benthic response, and that both duration and exposure to varying low oxygen conditions as well as the recovery period of oxygen to > 5 mg L⁻¹ are important to benthic habitat health. While the duration of exposure to DO from < 2 mg L⁻¹ to near anoxia remains an important factor in benthic health, benthic organisms can better tolerate periods of low oxygen when reoxygenation occurs after a short time interval. More research is needed to better quantify the relationship between oxygen stress and recovery on benthic habitats, particularly in systems where low DO exposure and recovery can vary over timescales of hours to days.

Measuring infaunal population dynamics relies on destructive sampling that disturbs sediments and removes animals from their habitat. Establishing effective, non-invasive sampling methods for monitoring infaunal populations can reduce the impact of scientific sampling and facilitate efficient population assessments. Using intertidal soft-shell clams (Mya arenaria L.) in eastern Canada, we explored whether population density and size structure could be estimated from visible siphon holes. Across four sites with varying sediment characteristics and infaunal species assemblages, we predicted the presence of M. arenaria with 78–100% accuracy by visually assessing siphon holes. Smaller holes (< 7.5 mm) were more likely to be misidentified. Siphon hole count was a strong predictor of actual clam count and biomass at most sites, except the site with wet muddy sediment and high densities of other infaunal species. Siphon hole length was positively related to clam shell length and wet weight at all sites; however, relationships typically had low R² values (< 0.35). Ultimately, visual assessments of intertidal siphon holes can be effective for estimating M. arenaria densities and size structure in some habitats. Testing the application of this method to other habitats and species is warranted.

Asymmetric tidal dynamics are of great significance for material transport and morphological evolution in estuaries. The tidal dynamics of the macro-tidal Hangzhou Bay (HZB) are characterized by flood-ebb asymmetries, spring-neap asymmetries, surface-bottom asymmetries, and up-downstream asymmetries. The mechanisms of spatio-temporal asymmetric tides and lateral flows in HZB were studied through a fully calibrated three-dimensional numerical model. The results show that tidal tides, tidal currents, and tidal asymmetry in HZB varied specially and temporally. In general, the bay was mostly flood-dominant. Temporally, tidal duration asymmetry in the bay fluctuated between spring and neap tides, with larger skewness during spring tides and smaller skewness during neap tides. The locally produced overtides are the primary sources of shallow-water tides in the bay, and the interaction between the lunar semi-diurnal tide M₂ and the solar semi-diurnal tide S₂ generates shallow-water overtides and deforms tidal asymmetries. The dissipated tidal energy may consumed by the bottom friction, with less passed to the generated shallow water overtides M₄ and M₆ tides (A_M4 = 12.07 cm, A_M6 = 3.91 cm) when comparing with the experiments that open boundary is purely forced by M₂ tide (A_M4 = 13.63 cm, A_M6 = 6.31 cm). The increased depth reduces the bottom friction and the convergence of volume, resulting in skewness values close to zero (γ_TDA = 0.220, γ_M2-M4 = 0.141, γ_M2-M4-M6 = −0.002, γ_M2-S2-MS4 = 0.105). The changes of tidal duration asymmetry caused by the increased channel convergence, reduced bay width, and reclaimed intertidal zone spatially vary in different parts. The bottom friction contributed to the generation of the shallow-water tides and asymmetries in the bay (the RVRs for M₄ and M₆ are −73.5% and −92.5%), while the Coriolis force (the RVRs for M₄ and M₆ are 4.8% and 8.9%) and nonlinear advection (the RVRs for M₄ and M₆ are −17.3% and − 21.8%) are minor factors. The findings of the study provide hydrodynamic foundations for the research of sediment transport and estuarine evolution in similar macro-tidal turbid estuaries worldwide.

Bottom-up and top-down controls regulate the structure and function of ecosystems through trophic resources and consumption pressure, respectively. The relative contributions of both controls over tropical sponges have been documented; however, it remains unknown how these controls regulate sponge populations in temperate environments. We focused on the globally distributed sponge Hymeniacidon perlevis inhabiting two tidal channels in San Antonio Bay (Argentine Patagonia) with different anthropogenic nutrient loads and experimentally tested the relative contribution of spongivores (i.e., sponge consumers) and trophic resources (i.e., dissolved inorganic nutrients and POC proxies) in sponge growth. The presence of spongivores was evaluated, as well as the relevance of trophic resource concentrations in the sponge abundance pattern. Hymeniacidon perlevis was more abundant (5.42% vs. 1.29% in cover), grew more (39.6% vs. −10.9% in volume, 89.5% vs 13.9% in surface area), and experienced less biomass reduction (−19.9% vs. −46.2% in dry weight) in the channel with the highest concentration of trophic resources compared to the non-enriched channel, while spongivores had a negligible effect. Among trophic resources, nitrate concentration was the one that best explained the abundance pattern of H. perlevis, with sponge cover changing by 1.02% for each µmol L⁻ change in nitrate concentration. Overall, our results show that the population of H. perlevis is mostly bottom-up controlled. The role of a microbial symbiotic pathway in the fulfillment of the nutritional requirements of H. perlevis is also discussed.

Graphical Abstract