A Survey of X2 Isohaline Empirical Models for the San Francisco Estuary

This work surveys the performance of several empirical models, all recalibrated to a common data set, that were developed over the past 25 years to relate freshwater flow and salinity in the San Francisco Estuary (estuary). The estuary’s salinity regime—broadly regulated to meet urban, agricultural, and ecosystem beneficial uses—is managed in spring and certain fall months to meet ecosystem objectives by controlling the 2 parts per thousand bottom salinity isohaline position (referred to as X2). We tested five empirical models for accuracy, mean, and transient behavior. We included a sixth model, employing a machine learning framework and variables other than outflow, in this survey to compare fitting skill, but did not subject it to the full suite of tests applied to the other five empirical models. Model performance was observed to vary with hydrology, year, and season, and in some cases exhibited unique limitations as a result of mathematical formulation. However, no single model formulation was found to be consistently superior across a wide range of tests and applications. One test revealed that the models performed equally well when recalibrated to a uniformly perturbed input time-series. Thus, while the models may be used to identify anomalies or seasonal biases (the latter being the subject of a companion paper), their use as inverse models to infer freshwater outflow to the estuary from salinity observations is not expected to improve upon the absolute accuracy of existing outflow estimates. This survey suggests that, for analyses that span a long hydrologic record, an ensemble approach—rather than the use of any individual model on its own—may be preferable to exploit the strengths of individual models.

Species richness and functional attributes of fish assemblages across a large-scale salinity gradient in shallow coastal areas

Coastal ecosystems are biologically productive and their diversity underlies various ecosystem services to humans. However, large-scale species richness (SR) and its regulating factors remain uncertain for many organism groups, owing not least to the fact that observed SR (SRobs) is strongly dependent on sample size and inventory completeness (IC). We estimated changes in SR across a natural geographical gradient using statistical rarefaction and extrapolation methods, based on a large fish species incidence dataset compiled from Swedish fish survey databases. The data covered nearly five decades (1975–2020), a 1,300 km north-south distance and a 10-fold salinity gradient along sub-basins of the Baltic Sea plus Skagerrak. Focusing on shallow coastal and offshore areas (< 30 m depth), we calculated standardized SR (SRstd) and estimated SR (SRest), and related these to sub-basin annual mean salinity and water temperature. IC was high, 98.5 %–99.9 %, in the 10 sub-basins with sufficient data for analysis. The recorded fish species were of 75 % marine and 25 % freshwater origin. Total fish SRobs was 144 for shallow coastal areas, and 110 for shallow offshore areas. Sub-basin specific SRest for coastal areas varied between 35 ± 7 (SE) and 109 ± 6 fish species, and was ca. three times higher in the most saline (salinity 29-32) compared to the least saline sub-basins (salinity 2.7). Completing information on functional attributes showed that differences along the salinity gradient reflected an increased share of coastal resident fish species in lower salinities, and a higher share of migratory fish at higher salinities. The proportion of benthic and demersal fish species was also lower in the least saline sub-basins, and increased with increasing salinity. If climate change lowers the salinity regime of the Baltic Sea in the future this may hence influence the SR and community composition of fish.

Effect of water salinity on survival and osmotic level of larval (Zoea Stage) of mud crabs Scylla tranquebarica

Mud crab of the genus Scylla are considered one of the most sought-after seafood today. This crab species has high quality and delicious aging growth rate and encourage expansion in the aquaculture sector especially in Southeast Asian Countries. However, salinity changes will cause changes in organisms osmotic pressure, and every aquatic biota has an optimal salinity range for survival. The study focuses on evaluating the effect of water salinity on the survival and osmotic levels of the purple mud crab, larvae of Scylla tranquebarica at the zoea stage. The LC50 assessment was performed in 10 different level of water salinity (0; 5; 10; 15; 20; 25; 30; 35, 40, and 50 ppt). Each treatment involved 20 ind./L of newly hatched crabs and being observed for 24 h in 10 different water salinity using 1 L volume glass container. The number of crab’s mortality were taken for each salinity regime. Larval behavior monitored during experiment. Meanwhile, the measurement of osmotic level was carried out at the salinity of 25, 30, and 35 ppt. The result shows that mud crab larvae exhibit any tolerance on the low salinity ranged from 0-10 ppt and the salinity of > 40 ppt. On the other hand, mud crab larvae were still able to survive at the salinity ranged from 20-40 ppt for more than 24 hours. The trend of the osmotic level of mud crab to survive is by hypo osmotic to iso osmotic.

STABILITY OF SMALL BARRIER ESTUARIES: FIRST PASS ASSESSMENT USING PUBLIC DATA AND ATTRACTOR MODELLING

Small barrier estuaries are common in temperate latitudes where catchment runoff is small. The entrance state controls the tidal exchange and hence the salinity regime and flushing. To aid the study and management of these estuaries, predictions of the stability and future evolution of the estuary entrance must be made. This paper demonstrates the application of the attractor method to determine the stability of a wide sample of estuaries on the south-eastern coast of Australia, using only data available in broad-based public data bases. The method uses a simple hydrodynamic-sediment balance model, run for thousands of scenarios and thousands of tide cycles to identify the long-term dynamic equilibria - the attractors. The model predictions are shown to match stability data in the data bases and to provide realistic predictions of the entrance evolution. The results have direct applicability to high-level assessment of coastal assets and to optimal selection of model scenarios for more detailed modelling of any selected estuary.

Dynamics of Reed Vegetation Area in the Sivash Gulf (Sea of Azov) according to Satellite Data

The paper analyzes peculiarities of formation of the Sivash Gulf coastal area, which manifest at the moment as a combination of abrasive and accumulation processes. The paper gives more details about vegetation coasts (a special coast type related to a specific sedimentation type) defined by reed vegetation development, which is prevalent near clough entries in apexes of ingressive gulfs and “secondary” lagoons. Using data of digital multizonal satellites Landsat 5 and 7 as well as Sentinel-2 L1C, the paper considers dynamics (2014–2020) of reed coast areas in the East and South Sivash after damming the North Crimean Canal in 2014 as a consequence thereof and related increase of salinity in the gulf water area. For comparison, the paper analyzes dynamics of vegetation coast spread as a result of the canal functioning, also reed vegetation area growth up to the 1990s is considered. The ecological role of reed vegetation as a distinctive biotope is shown. It is emphasized that damming of the North Crimean Canal not only led to changes of the gulf salinity regime but also affected its entire ecosystem, including the reed coast, formation of which was promoted by increase of fresh water volume in the gulf water balance. Impact of the North Crimean Canal and its damming in 2014 on change of the reed vegetation area in the East and South Sivash was estimated using satellite data. At the studied zone of Prisivashie, flood plain areas shrank most of all in the Knyazevsky and Balaganovsky Gulfs as well as near the Alekseevskaya extinct lake in the South Sivash. In the Rogachinsky Gulf, despite discontinued drainage, influence of waste waters remains significant. Therefore, reed communities have preserved here best. It is shown that for some water areas shrinkage of areas populated by reed vegetation can considerably slow down provided there are local p ermanent or periodic fresh water sources, e. g. waste waters coming from settlements.

The Response of Estuarine Ammonia-Oxidizing Communities to Constant and Fluctuating Salinity Regimes

Aerobic nitrification is a fundamental nitrogen biogeochemical process that links the oxidation of ammonia to the removal of fixed nitrogen in eutrophicated water bodies. However, in estuarine environments there is an enormous variability of water physicochemical parameters that can affect the ammonia oxidation biological process. For instance, it is known that salinity can affect nitrification performance, yet there is still a lack of information on the ammonia-oxidizing communities behavior facing daily salinity fluctuations. In this work, laboratory experiments using upstream and downstream estuarine sediments were performed to address this missing gap by comparing the effect of daily salinity fluctuations with constant salinity on the activity and diversity of ammonia-oxidizing microorganisms (AOM). Activity and composition of AOM were assessed, respectively by using nitrogen stable isotope technique and 16S rRNA gene metabarcoding analysis. Nitrification activity was negatively affected by daily salinity fluctuations in upstream sediments while no effect was observed in downstream sediments. Constant salinity regime showed clearly higher rates of nitrification in upstream sediments while a similar nitrification performance between the two salinity regimes was registered in the downstream sediments. Results also indicated that daily salinity fluctuation regime had a negative effect on both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) community’s diversity. Phylogenetically, the estuarine downstream AOM were dominated by AOA (0.92–2.09%) followed by NOB (0.99–2%), and then AOB (0.2–0.32%); whereas NOB dominated estuarine upstream sediment samples (1.4–9.5%), followed by AOA (0.27–0.51%) and AOB (0.01–0.23%). Analysis of variance identified the spatial difference between samples (downstream and upstream) as the main drivers of AOA and AOB diversity. Our study indicates that benthic AOM inhabiting different estuarine sites presented distinct plasticity toward the salinity regimes tested. These findings help to improve our understanding in the dynamics of the nitrogen cycle of estuarine systems by showing the resilience and consequently the impact of different salinity regimes on the diversity and activity of ammonia oxidizer communities.

DISTRIBUTION AND ABUNDANCE OF PHYTOPLANKTON IN VEMBANAD ESTUARY, A RAMSAR SITE ON SOUTH WEST COAST INDIA

The phytoplankton abundance and distribution from the Vembanad estuary, south west coast of India was studied for a period of two years from March, 2011 to February, 2013. A total of 73 genera of phytoplankton were recorded and Bacillariophyceae formed the dominant group in all seasons of the study. The constant addition of nutrients particularly, nitrate and phosphate based fertilizers used in the Kuttanad paddy fields enhanced the phytoplankton production in the estuary. During 2011-2012 and 2012-2013 period, phytoplankton biomass varied from 2 to 46 ml/m3 (av. 11.09 ml/m3) and 1 to 26 ml/m3 (av. 5.78 ml/m3) respectively. Phytoplankton biomass, distribution and species composition showed variations in contrast to the prevailing water temperature, light intensity (Secchi disc visibility), nutrient availability, grazing pressure, tide and water movements, and seasonal pattern and even with time of day. The salinity regime of the water body also influenced the phytoplankton distribution in the study locations. Phytoplankton production showed a higher value in the northern as compared to the southern zone. Thus, the mesohaline condition prevailing in the northern zone of the estuary supported higher abundance and diversity of phytoplankton species which in turn contributed to higher production rate.