Salinity Minimum Research Articles

Distributed salinity sensor based on optical frequency domain reflectometry with polyimide coated single mode fiber

A distributed optical fiber sensor for salinity sensing is proposed and analyzed. The sensor uses a single mode fiber coated with polyimide and is based on optical frequency domain reflectometry. By applying the polyimide solution to the surface of the fiber, a layer of polyimide film is formed by heating. As the polyimide coating is very sensitive to changes in the salinity of the external solution, the polyimide film will expand or shrink as the external concentration changes. This converts the value of salt concentration into the frequency shift in the spectrum through cross-correlation. To characterize the sensor, four polyimide-coated sensors with different average coating thicknesses of 126 µm, 170 µm, 192 µm and 249 µm were fabricated. The sensitivities of -21.71 GHz/(mol/L), -28.55 GHz/(mol/L), -29.97 GHz/(mol/L) and -42.78 GHz/(mol/L) were achieved when the salinity was measured from 0 mol/L to 3.09 mol/L, respectively. The sensitivity and response time of polyimide fibers with different diameters were measured. The minimum salinity measurement uncertainty of the sensor is 0.05 mol/L. The sensor has high sensitivity and has promising applications in observing ocean parameters and ion chemical sensing.

Satellite-Derived Variability of Sea Surface Salinity and Geostrophic Currents off Western Patagonia

The coastal ocean off western Patagonia is one of the main coastal regions with high freshwater inputs from rivers, rain, and glaciers in the Southern Hemisphere. This study conducts an analysis of the seasonal and interannual variations in sea surface salinity and meridional geostrophic transports, specifically focusing on the Cape Horn Current, using improved satellite-derived data of sea surface salinity (SSS) and geostrophic velocities spanning an ∼11-year period (September 2011–August 2022). Our results reveal a clear salinity minimum in a coastal band between 42–54°S associated with the highest freshwater content. The average geostrophic currents are stronger south of 49°S, in line with the location of the Cape Horn Current. The average salinity minimum tends to disappear south of 54°S, with salinity values increasing slightly southward. The seasonal cycle of salinity shows the most pronounced minimum in summer (∼33.2–33.4). The greatest variability in salinity (standard deviation of salinity fields) occurs in the southern region of the Cape Horn Current. Hovmöller plots reveal two cores of minimum salinity observed in spring and summer (∼33.3–33.4). The freshwater off the Gulf of Penas contributes to the northern core. The meridional geostrophic transport differs between the northern and southern sections, with transports predominantly towards the Equator (Pole) north (south) of about 47–48°S during spring–summer. There is a marked seasonal variability in the magnitude and northern limit of the southward-flowing Cape Horn Current, being extended further north during winter and with a maximum average magnitude during summer–fall (about −2×104 m2 s−1). On the interannual scale, a major drop in surface salinity occurred off northern and central Patagonia during 2018–2019. Finally, a potential long-term freshening trend is observed in the coastal area off southern Patagonia (south of 52°S), although prolonged data records are essential to confirm this pattern.

Effect of Salinity Variation on the Survival Rate of Amphiprion ocellaris (Common Clownfish)

This study investigates the impact of changes in salinity on the survival rate of Amphiprion ocellaris, commonly known as the Common Clownfish. The aim is to ascertain the minimum salinity threshold that clownfish can tolerate, thereby offering valuable insights for professionals and researchers in the field of ornamental fish. This study utilized ten fully-grown clownfish, all of which were over one year old and had an average length of 38 mm and weight of 1.50 g. Every fish was placed in a carefully regulated tank where the salinity was systematically reduced by around 3 parts per thousand (ppt) every week, beginning from an initial salinity of 32.42 ppt. The weekly monitoring involved the assessment of water quality indices, including dissolved oxygen (DO), temperature, and pH. Clownfish exhibited indications of distress, such as reduced appetite and depigmentation, when exposed to a salinity of 8 parts per thousand (ppt). Death occurred when the salinity was decreased to 7 ppt, indicating their incapacity to survive below 10 ppt. These findings suggest that common clownfish are not appropriate for freshwater environments and necessitate specific salinity levels in order to flourish. These findings have practical consequences for the ornamental fish industry, underscoring the significance of maintaining suitable salt levels to avoid misinformation and financial setbacks. Additional investigation should focus on the possibility of raising clownfish in settings with reduced salinity to improve their ability to adapt and better aquaculture techniques.

Connecting subtropical salinity maxima to tropical salinity minima: Synchronization between ocean dynamics and the water cycle

Recent satellite observations reveal that the annual cycles of tropical sea surface salinity (SSS) minimum (Smin) and subtropical SSS maximum (Smax) are phased locked, with the former leading the latter by six months. The evidence suggests an interconnected nature between the salinity extrema, motivating an investigation into the underlying mechanisms. It is found that the poleward Ekman transport, driven by trade winds, stands as a pivotal conduit linking the influence of tropical freshwater to high salinities in the subtropical oceans. Two key aspects are central to the operation of this advective oceanic pathway. One is the annual formation of the near-equatorial Smin zones by Ekman convergence of the freshwater sourced from the double Intertropical Convergence Zones. The other is the advection time taken by Ekman transport traveling at an average speed of approximately 7 cm s−1. This speed allows the near-equatorial salinity anomalies to be carried to the subtropical Smax fringes in six months, aligning closely with the observed semi-annual phase shift between Smin and Smax. During El Niño - Southern Oscillation (ENSO) warm and cold episodes in recent decade, poleward propagation of near-equatorial low salinity anomalies was particularly prominent, leading to substantial surface freshening in the subtropical Pacific. Interestingly, the latitudinal spread of the ENSO-triggered SSS anomalies is confined within the domain of the poleward Ekman transport. This confinement reaffirms the inherent connection between the tropical Smin and subtropical Smax regions, shedding light on the intricate interplay between oceanic processes, the water cycle, and SSS distributions.

Experimental and field evidence suggests extreme salinity tolerances in Coxiella gastropods from Australian salt lakes

This study aimed to determine salinity tolerances in Coxiella gastropods from Australian salt lakes and whether different species exhibit characteristically different tolerances. Controlled gradual accumulation experiments were conducted to estimate both the maximum and minimum salinity levels at which 50% of individuals (IC50) remained active for 25 populations representing six species. All studied species showed remarkable euryhalinity and were tolerant of very high levels of salinity, some more than others, while minimum salinity tolerance varied little among populations and species. The experimental trends in salinity tolerances were consistent with the salinity distributions of species in the field, although the former were typically broader than latter. The findings suggest that Coxiella comprises some of the most salt tolerant gastropods globally.

The role of the low-salinity, high-nitrate layer as a novel lateral mechanism for nutrient supply into the San Jorge Gulf (Patagonian Shelf) during mid-summer

The San Jorge Gulf (Patagonian Shelf) is productive in the surface layer mainly in three regions during summer associated with small-scale coastal fronts controlled by interaction with islands to the NE, with coastal upwelling to the SW due to the wind, and with the dynamics of the Southern Tidal Front at the gulf mouth. In early February 2014, a time series of the water column characteristics were performed with a Conductivity-Temperature-Depth (CTD)/rosette package and a vessel hull-mounted acoustic Doppler current profiler (ADCP) over two complete semidiurnal cycles in a quasi-fixed location of the San Jorge Gulf mouth in order to evaluate interaction mechanisms between the surface mixed layer and interior layers that condition the distribution of properties. The pycnocline thickness defined by 24.90 kg m−3<σ0< 25.50 kg m−3 is driven by the semidiurnal tide, with a mean modulation of 32.7 m amplitude during a 2.75 m sea-level oscillation (R2 = 0.75). Patchy cores of minimum salinity (SP∼33.10) with high nitrate concentrations (9.9 mmol m−3, on average) were found at the pycnocline centre (38.0 m ± 4.5 m) during the flood as pulses of sub-surface water between consecutive high tides. Also, a high correlation (R2 = 0.88) was found between the thickness of the low-salinity layer (SP< 33.20) and the pycnocline thickness. A clear mode-1 internal tide of semidiurnal frequency was identified during the time series with some non-linear features in the first tidal cycle. As a consequence, a difference in the reversal of the internal tide was seen during both tidal cycles. The water column decreases its stratification when the low-salinity water enters the gulf due to a widening of the pycnocline thickness. At the same time, moderately high nitrate concentrations are observed in the surface mixed layer, suggesting a novel lateral mechanism to maintain the surface chlorophyll layer observed during mid-summer.

Distribution of water masses in the tropical-subtropical convergence off Mexico, using an autonomous profiler (2017–2021).

Data from an autonomous ARGO float launched on December 26, 2016, south of the entrance of the Gulf of California, reveal variations in the distribution of water masses in the tropical-subtropical convergence off Mexico. The float trajectory drifted through three areas of distinct characteristics: i) the southern Gulf of California in January 2017, ii) south of Cabo San Lucas from February 2017 to January 2018, when the float crossed an eddy, and iii) off Cabo Corrientes from February 2018 to April 2021. During the sampling, the vertical distribution of the water masses showed for the first time with “in situ” data a semiannual variability mainly in the salinity as response to poleward propagating semiannual Kelvin wave. The surface water masses had potential densities <26 kg m−3. A shallow salinity minimum (<34.6 g kg−1) observed at ∼ 70 m depth south of Cabo San Lucas provided a proxy for the California Current Water distribution, which had its maximum southern extension during winter. The Tropical Surface Water was observed off Cabo Corrientes mainly above 50 m depth and had maximum northern extension during summer. The Gulf of California Water was scarcely evident in the float trajectory in the southern Gulf. The Transitional Water was located above ∼ 50 m depth and was dominant southwest of Cabo San Lucas during winter and spring, when the California Current Water extended furthest south. The Subtropical Subsurface Water, in spite of its high density (>25 kg m−3) at times extended up to only 20 m below surface in the three areas around the summer as part of a semiannual cycle. The intermittent presence of this oxygen-deficient water close to the surface (∼ 20 m depth) is consistent with the on-going expansion of the shallow oxygen minimum zone. Wavelet analysis highlighted a bimonthly spectral frequency in the StSsW lower limit when the float crossed the eddy in the south of Cabo San Lucas, showing the influence of the mesoscale features of the area, and therefore a possible interchange of properties between deep waters.

Limited population genetic variation but pronounced seascape genetic structuring in populations of the Mediterranean mussel (Mytilus galloprovincialis) from the eastern Adriatic Sea.

Population genetic analysis of variation at five neutral microsatellite loci for Mediterranean mussels (Mytilus galloprovincialis) from 18 sites along the eastern Adriatic Sea revealed little or no spatial variation. In contrast, seascape genetics analysis revealed a pronounced locus-specific gradient in allelic and genotypic frequencies across the study region. At a sixth locus, MGE7, the frequencies of two alleles, MGE7243 and MGE7249, were strongly associated, negatively and positively, respectively, with a single environmental variable - minimum salinity (minSAL). The frequency of the MGE7243/243 homozygous genotype was strongly negatively associated with minSAL, whereas the frequencies of the MGE7246/249 and the MGE7249/249 genotypes were strongly positively correlated with minSAL. Interpretation of these pronounced gradients is confounded by the fact that minSAL and another environmental variable, maximum sea surface temperature (maxSST), are highly correlated (R=-.911) and are therefore not necessarily acting independently. BLAST searches of the MGE7 locus against M. galloprovincialis whole genome shotgun sequence returned an alignment with contig mg10_S01094 (accession UYJE01010330.1) and 7 predicted M. galloprovincialis proteins VDI82194.1 - VDI82200.1. Conserved domain searches revealed a similar structure to the transcriptional regulator Msx2-interacting protein. The BLASTp search also returned significant alignments to Msx2-interacting proteins in Mytilus coruscus, Crassostrea virginica, and Haliotis rubra. The existence of the MGE7 gradient highlights the role that environmental variation may play in retarding gene flow among wild M. galloprovincialis populations, and also how the success of collection of young mussels (spat) from one site and their transfer to another site (the farm) may be influenced by a single factor such as minSAL or maxSST on a localized scale.

Seasonal variation of mixed layer depth and thermocline thickness from the ctd argo float data in the Southern Makassar Strait

The Makassar Strait (MS) is directly connected to the Sulawesi Sea in the north and the Java Sea and the Flores Sea in the south and is the main route of Indonesia Throughflow(ITF). Hence Stratification of water mass in the MS is influenced by the dynamics of these waters. This study aims to describe seasonal variations of the mixed layer depth and the thermocline thickness, identify ITF water masses, and the occurrence of coastal upwelling in the southern part of the MS. The results show that the North Pacific water origin of North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW) are dominant, with a maximum salinity characteristic of 34.6-34.7 psu in the thermocline layer and a minimum salinity of 34.5 psu in the intermediate layer. However, near-surface fresh Java Seawater appears to be dominant during the northwest monsoon (NWM-DJF) period. Seasonal variation of the mixed layer depth is much deeper (93m) and vertical gradient of temperature and salinity are much stronger due to the presence of fresh Java Sea water advected by the eastward Java Monsoon Current. However, during the southeast monsoon (SEM-JJA) period the mixed layer depth is much shallower and the thermocline layer is much thicker (198 m), Associated with much stronger ITF and upwelled colder water from the upwelling area. The coastal upwelling event during the SEM period in the study are is characterized by an increase of isotherm of 25 °C, isohaline of 34.0 psu, and isopycnal of 22.0 kg/m3 from about 75 m depth to the sea surface layer.

Low Sea Surface Salinity Event of the Japan Sea During the Last Glacial Maximum

AbstractAbnormal lightening of the oxygen isotope ratio (δ18O) of planktonic foraminifera during the Last Glacial Maximum (LGM, ∼21 kyr BP) suggests that the Japan Sea had experienced a low sea surface salinity event at that time. However, the exact value and timing of minimum salinity have been controversial so far. To address this issue, we adopt a simple box model and reconstruct the sea surface salinity in the Japan Sea (SJP) over the past 35 kyr with a focus on the LGM period. In particular, as input data for the box model, the inflow transport through the Tsushima Strait (Q) is converted from sea level evolution using a newly defined relationship, in which Q reduces non‐linearly with the sea level reduction through a dynamically‐constrained realistic ocean model. Meanwhile, another input data of the box model, sea surface freshwater flux (precipitation minus evaporation (P‐E) evolution), is obtained by averaging multi‐paleoclimate models (PMIP3 and MIROC4m models) results. The reconstructed SJP using the box model reached its minimum value (20.2) at 20 kyr BP with a high coefficient of determination (R2) for δ18O (0.81, p &lt;&lt; 0.01). Further analysis demonstrates that the above non‐linear relationship, determined by h3/2 (h is the strait depth), promises a more reasonable reconstruction of the SJP evolution. It is also concluded that both the value and timing of the minimum SJP depend on the Q evolution, and the P‐E evolution can modify the former. Therefore, the combination of Q and P‐E determines the exact value and timing of minimum salinity.

Anatomical study of the three-dimensional structure of a supercharged cold eddy generated in the Kuroshio Extension

The three-dimensional structure of a supercharged cold eddy, which showed strong temperature anomaly, was continuously investigated for 83 days based on current- and pressure-equipped inverted echo sounder observations in the Kuroshio Extension region. The eddy was generated on December 9, 2004, shed from the Kuroshio approximately 30 days later, and moved out of the observation area on March 1, 2005. During the stable period, the eddy had a radius of approximately 60–80 km, a depth of approximately 3,000 m, and a westward speed of 7.4 km/d. The maximum temperature anomaly in the eddy center reached -9.1°C at 360 dbar, whereas the minimum (maximum) salinity anomaly reached -0.68 (0.20) psu at 340 (780) dbar. Under the stream function coordinate, the kinetic energy of the eddy first increased and then decreased from the center to boundary, whereas the vorticity decreased overall. Energy budget analysis showed that eddy energy mainly originated from the Kuroshio during eddy formation by advection, whereas the baroclinic conversion (BC) and barotropic conversion (BT) played a dissipative role. After the eddy had been completely separated from the Kuroshio, the mean flow energy was transferred to eddy energy through BC and BT, which further enhanced eddy potential energy and eddy kinetic energy.

Spiciness anomalies in the upper North Pacific based on Argo observations

The density-compensated salinity anomalies (spiciness anomalies) in the upper North Pacific were investigated using Argo float profiles during 2004–2018. The freshening of the subtropical thermocline was found within the Central Mode Water (CMW) and the North Pacific Intermediate Water (salinity minimum). Meanwhile, the increase of salinity was found in the lighter layer within the North Pacific Tropical Water (salinity maximum). The interannual and longer spiciness anomalies were interpreted with three-dimensional evolutions and were linked to the fate of three mode waters (i.e., Subtropical Mode Water (STMW), Eastern Subtropical Mode Water (ESTMW), and CMW) for the first time. In the STMW, the salinity was dominated by a quasi-decadal variability, which was consistent with the KE variability with a 1-year lag, and did not show rapid freshening. In the CMW, the salinity decreased with a quasi-decadal variability, which was weaker and out of the phase compared with the STMW. In the ESTMW, the salinity was dominated by year-to-year variability. The spiciness anomalies originated mainly in the outcrop region of the isopycnals, where they were accompanied by the formation and the subduction of the mode waters. They also propagated and decayed downstream the geostrophic currents. However, a few of the interannual anomalies found in the northern part of the CMW were almost dampened before their spread further south. In addition, some anomalies in the ESTMW appeared and intensified without a connection to the mixed layer. These anomalies occurred far from the outcrop line, suggesting these were caused likely by salt fingering associated with the modification of the mode waters. Furthermore, the propagation of the spiciness anomalies in the western to the central subtropics was significantly faster than the geostrophic current and inclined to the inner side of streamlines. These indicated the transport by eddies in addition to the mean geostrophic currents.

Properties, sensitivity, and stability of the Southern Hemisphere salinity minimum layer in the UKESM1 model

Antarctic Intermediate Water (AAIW) is a water mass originating in the Southern Ocean characterised by its low salinity. The properties of the salinity minimum layer that characterise AAIW in the CMIP6 UKESM1 model and its response to different climate change scenarios are investigated. In UKESM1, the depth of the salinity minimum shoals by 116 m in the SSP5-8.5 run compared to the control run by 2080–2100. The salinity minimum also gets warmer (+ 1.9 °C) and lighter (− 0.4 kg/m3) and surface properties where the salinity minimum outcrops warm, freshen and lighten in all scenarios. In spite of these expected changes in properties, the location where the salinity minimum outcrops does not change in any of the future scenarios. The stability of the outcrop location of the salinity minimum is linked to the relative stability of the position of the Antarctic Circumpolar Current (ACC) in UKESM1. The position of the ACC does not follow the maximum wind stress trend, which intensifies and shifts poleward under radiative forcing. Changes in surface buoyancy fluxes in the region are consistent with the changes in hydrographic properties observed at depth on the salinity minimum mentioned above. However, transformation rates at the density corresponding to the salinity minimum outcrop remain constant in all scenarios. Stability in transformation rates at that density is due to the haline and thermal contributions counteracting one another. This analysis identifies two features (outcrop location, transformation rate) associated with the salinity minimum defining AAIW that show remarkable stability in an otherwise changing world. The effect of model resolution and other parameterisations on these findings have yet to be evaluated.

A Field Validated Model of Temporal Variability in Oyster Habitat Suitability

This work presents the development and validation of a spatially and temporally variable oyster habitat suitability model for the western Mississippi Sound, northern Gulf of Mexico. In the work, we (1) develop an oyster habitat suitability model based on existing conditions in 1 year, (2) forecast habitat suitability throughout the same location in following years, and (3) validate the model using independent data describing field counts of live and recent dead oysters in those following years. The model uses four environmental factors to determine habitat suitability, namely: maximum annual temperature, maximum annual salinity, minimum annual salinity, and minimum annual dissolved oxygen. Overall, the model does not discriminate well between good and poor habitat when the habitat suitability score is less than 0.2; however, when the habitat suitability score is greater than 0.2, there is a high confidence that are more live than recent dead oysters. The results also show that habitat suitability varies by up to 0.45 in any single location (one standard deviation; on a scale from 0 to 1). This is important for evaluating which areas will be most resilient for oyster habitat under a variety of conditions. This study presents the first validated statistical model of temporally and spatially varying oyster habitat suitability.

Labrador Slope Water connects the subarctic with the Gulf Stream

Labrador Slope Water (LSLW) is a relatively fresh and cool water mass that originates from the Labrador Current in the subarctic and is known to occur in the Eastern Slope Sea on the US-Canadian shelf-slope north of the Gulf Stream. It has potential densities of 27.4–27.65 kg m−3. Using ocean observations, we show here that the LSLW penetrates as a boundary current deeply into the Western Slope Sea (west of 66°W) as a salinity minimum between 400 and 600 m, bringing it into close proximity with the Gulf Stream. The LSLW at Line W (near 69°W) also spreads across, and brings fresher and thicker waters to, the Slope Sea north of the Gulf Stream. A high-resolution ocean model simulation shows that the spreading of the LSLW occurs throughout the entire Slope Sea through the extrusion of fine-scale filaments from the boundary current following interaction with Gulf Stream meanders and eddies. At Line W, the LSLW is also found to be fresher and thicker between 2003 and 2008, when the Atlantic Meridional Overturning Circulation (AMOC) at 26°N is higher (by 3 Sv), and the Shelf Slope Front is further south (by 0.7°), compared to AMOC low conditions in 2009–2014. The thicker LSLW causes lighter isopycnals to rise over the shelf slope, and through increasing the lateral density gradient contributes an additional 1.3 Sv to the Gulf Stream transport. These changes to the LSLW and the Shelf Slope Front are likely to result from an enhanced flow of the Labrador Current into the Slope Sea, caused by changes in the wind stress in the subpolar gyre. The transport of the LSLW (as opposed to the deeper Labrador Sea Water) thereby offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting changes in the subarctic with subsequent variability in the Gulf Stream and AMOC.

Reappraisal of archaeal C20-C25 diether lipid (extended archaeol) origin and use as a biomarker of hypersalinity

The diether core membrane lipid sesterterpanyl-phytanyl-glycerol (so-called extended archaeol and often abbreviated C20-C25) is considered as a hallmark of Haloarchaea, a clade of archaea thriving under extreme high salinities. We here report about extended archaeol occurrence in different saline aquatic settings with salinity ranging from ca. 50 psu (5 % NaCl w/v) to saturation (ca. 350 psu). This demonstrates that this lipid is not restricted to extreme saline environments but suggests a minimum salinity threshold of ca. 50 psu above which C20-C25 is most commonly produced. The proportion of C20-C25 relative to that of archaeol (C20-C20) did not appear linearly dependent on the salinity of the site and was potentially also influenced by pH and temperature, preventing its direct use as a quantitative salinity proxy based on the present data set. An extensive literature review of archaeal membrane lipid compositions further highlighted that taxonomy also contributes to the distribution of this lipid in the environment and identifies Natrialbales (one of the three orders of Haloarchaea) as the main source. Statistical analysis showed that, among Haloarchaea, C20-C25 producers display pH and salinity growth optima slightly higher than non-producers and are distributed within two distinct groups, one composed mostly of neutrophiles and one of alkaliphiles. In contrast, the presence of C20-C25 was not correlated to the optimal growth temperature of the strains. This suggests that two confounding parameters, i.e., taxonomy and adaptation to changes in salinity and/or pH, contribute to the distribution of C20-C25 within Haloarchaea.

Formation and Evolution of a Freshwater Plume in the Northwestern Tropical Atlantic in February 2020

AbstractIn February 2020, a 120‐km‐wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7°N/54°W‐56°W). It was initially stratified in the upper 10 m with a freshwater content of 2–3 m of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13–16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off‐shelf flow lasted 10 days, contributing to a plume area extending over 100,000 km2 and associated with a 0.15 Sv (106 m3 s−1) freshwater transport. The off‐shelf plume was steered northward by a North Brazil Current ring up to 12°N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favorable wind direction closer to the Amazon estuary, which contributed to north‐westward freshwater transport on the shelf. Other such events of freshwater transport in January–March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favoring the north‐westward freshwater transport toward the shelf break.

Extreme changes in salinity drive population dynamics of Catostylus mosaicus medusae in a modified estuary

Modifications to estuaries through the construction of barrages alter the natural dynamics of inhabitant species by controlling freshwater inputs into those systems. To understand the effects of modified freshwater flows on a native scyphozoan jellyfish, Catostylus mosaicus, and to identify the environmental drivers of medusa occurrence, we analysed a 20-year observational dataset composed of 11 environmental variables and medusa presence/absence from 15 sampling stations located below the Fitzroy Barrage, in the Fitzroy River, Queensland. Major decreases in salinity (minimum salinity 0) occurred approximately 16 times during the 20-year period and medusae disappeared from the estuary following every major freshwater flow event. Salinity was identified as the most influential variable contributing to variation in the number of upper estuary sites reporting jellyfish. We then ran two laboratory experiments to test the following hypotheses: (i) prolonged decreases in salinity impair survival, pulsation, and respiration rates of C. mosaicus medusae; and (ii) transient decreases temporarily impair pulsation and respiration but medusae recover when salinity returns to normal levels. Medusae were unable to survive extended periods at extreme low salinities, such that they would experience when a barrage opens fully, but had significantly higher survival and recovery rates following smaller, transient changes to salinity that might occur following a moderate rainfall event. This demonstrates for the first time that modification of freshwater flow by a barrage regulates the population dynamics of an estuarine jellyfish, and highlights the need for robust, long term datasets, and to firmly embed experimental approaches in realistic ecological contexts.

Effects of river discharge and the California Current on pycnocline depth at the eastern entrance to the Gulf of California

Monthly hydrographic data were obtained at one station for 4 years at the eastern entrance to the Gulf of California to determine the influence on the pycnocline of waters from river discharge or from the California Current. Data were collected from 2005 to 2009, 20 km offshore over a depth of 127 m on the continental shelf in the vicinity of the Tropic of Cancer. The 4-yr average of the maximum surface temperature was 30.01 °C and the minimum was 21.07 °C, with an absolute minimum of 18.32 °C in January 2008. For salinity, the 4-yr average of the maximum salinity was 35.35 g/kg. Most remarkably, an absolute minimum salinity of 31.08 g/kg was recorded in October 2018 due to freshwater runoff. Waters from the California Current were detected between 35 and 105 m depth from August to September during the four years of observations. Anomalies in sea surface height, with amplitude of centimeters, were associated to pycnocline base oscillations (isotherm of 18 °C) with an amplification factor of 250. The isotherm of 18° was correlated (r = -0.9) to the coefficients of the first empirical mode, which explained 90% of the temperature variance.

Juvenile growth deficit as an early alert of cockle Cerastoderma edule mortality

In the population dynamics of bi-phasic marine invertebrates, the fitness of one stage in the life of a cohort affects that of the following stage. This effect makes the prediction of the fate of a cohort challenging. We conducted a 22 yr monthly survey of a population of cockles Cerastoderma edule focussed on the juvenile stage. Two types of years were distinguished in terms of cohort longevity: a low-mortality group (‘L’) and a high-mortality group (‘H’). The cockle shell lengths of the 0+ cohorts was higher in the L group, in August and September. A cohort could hereby confidently be assigned to the L or H group as soon as August. Maximum cockle abundance in the cohorts, parasite load, or date of recruitment did not discriminate the groups, while an early date for the peak of the 0+ cohort abundance could be related to the L group. The maximum air temperature and the chl a concentration in July were higher in the H group, whereas the minimum salinity during the month of September was slightly lower in the H group of years. Therefore, a juvenile shell length under a given threshold was identified as an early alert for a short lifespan of the cockle cohort. This long-term analysis contributes to the deeper understanding of the population dynamics of bi-phasic invertebrates. The success of juveniles is not solely related to the intensity of recruitment or mortality by predation, but could also be linked to their fitness, their growth rates appearing as a proxy.