Response of a Small River Plume on Wind Forcing

Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (∼10 m) and temporal (∼1 min) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10–20 min). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.

Large River Plumes Detection by Satellite Altimetry: Case Study of the Ob–Yenisei Plume

Satellite altimetry is an efficient instrument for detection dynamical processes in the World Ocean, including reconstruction of geostrophic currents and tracking of mesoscale eddies. Satellite altimetry has the potential to detect large river plumes, which have reduced salinity and, therefore, elevated surface level as compared to surrounding saline sea. In this study, we analyze applicability of satellite altimetry for detection of the Ob–Yenisei plume in the Kara Sea, which is among the largest river plumes in the World Ocean. Based on the extensive in situ data collected at the study area during oceanographic surveys in 2007–2019, we analyze the accuracy and efficiency of satellite altimetry in reproducing, first, the outer boundary of the plume and, second, the internal structure of the plume. We reveal that the value of positive level anomaly within the Ob–Yenisei plume strongly depends on the vertical plume structure and is prone to significant synoptic and seasonal variability due to wind forcing and mixing of the plume with subjacent sea. As a result, despite generally high statistical correlation between the ADT and surface salinity, straightforward usage of ADT for detection of the river plume is incorrect and produces misleading results. Satellite altimetry could provide correct information about spatial extents and shape of the Ob–Yenisei plume only if it is validated by synchronous in situ measurements.

Effects of metal toxicity on the early life stages of the sea urchin Evechinus chloroticus

<p>Metals are a common source of pollution in coastal waters, and have long been recognised as a major concern for many marine species, especially their early life stages. Although effects have been examined using standard toxicity assays, the impact of metals in more complex and realistic exposure regimes is still poorly known, in particular with regards to latent effects across multiple life stages and the interaction of multiple stressors. In this thesis, the effects of metals were investigated for multiple life stages of the endemic New Zealand sea urchin Evechinus chloroticus. Standard short-term bioassays were performed on the early life stage of E. chloroticus and also the endemic abalone Haliotis iris, for comparison. These assays evaluated the toxicity of three major pollutants (copper, lead and zinc) alone and in combination, on these species. Embryos of both species were highly vulnerable to copper (EC50s: 5.4 and 3.4 µg/L respectively for E. chloroticus and H. iris) and zinc (27.7 and 13.1 µg/L) but relatively tolerant to lead (52.2 and 775 µg/L), and there was no evidence of synergistic effects of metal mixtures. The latent effects of copper across two life stages in E. chloroticus, larval and juvenile, were investigated with laboratory experiments using realistic scenarios of low copper concentration, short pulses of exposure and examining exposure through dietary intake as well as waterborne exposure. Strong latent and carry-over effects were observed even at low concentration and short exposure time. For example, individuals exposed as larvae to 10.4 µg/L Cu for two days developed normally during the larval stage but had strongly impaired subsequent growth, with average body size decreasing by 24% in the 25 d following settlement. Moreover, juveniles previously exposed to copper as larvae were less resistant to a subsequent exposure, with up to four times higher mortality. Latent effects were especially important when copper was present in the diet rather than dissolved in water. For example, E. chloroticus larvae exposed to 2.3 µg/L Cu in water and fed with an algal diet cultured in the same concentration had a settlement success three times lower than those exposed only to waterborne copper. Furthermore, a short pulse exposure (4 days) to copper in the algal diet was generally more toxic than chronic exposure, showing that a short-lived bloom of contaminated phytoplankton may have a more severe impact on zooplankton than chronic pollution. Because metal discharge in coastal water is generally associated with freshwater (e.g. storm water or river plumes), the toxicity of copper was evaluated in both normal and low salinity seawater. Low salinity (24 ppt) increased copper toxicity in E. chloroticus larvae under chronic exposure to high levels (15 µg/L; 43% and 80% lower survival and normal development rate, respectively) but not under a single pulse exposure (4 days) to low concentration (5 µg/L). This highlights the importance of using realistic exposure in laboratory assays. Finally, the effect of copper on adult E. chloroticus and in particular on their fertilisation success was evaluated. Strong sublethal effects were observed after exposure to 50 µg/L Cu for two weeks including spawning impairment (especially in females) and elevated copper burden in gonads (25-times higher than control animals). However, the fertilisation success of successfully spawning males was not affected. The prevalence of local metal contamination was also measured at the mouth of local river plumes and in E. chloroticus gonads at sites expected to vary in likely exposure to pollution. Copper levels exceeding water quality criteria were found in two instances in coastal agricultural runoff (Makara stream). Other metals were within water quality cirteria in all samplings. Adult E. chloroticus had an elevated copper burden in gonads in an urban site compared to a control site (0.77 µg/g vs. 0.27 µg/g). In total, this research demonstrates the need for considering toxic effects across multiple life stages and using realistic exposure regimes (e.g. timing, concentration, multiple stressors) to better understand the likely impact of metal pollution on marine populations. It also provides the first measure of metal toxicity on early life stages of an endemic species of cultural and commercial importance in New Zealand.</p>

Effects of metal toxicity on the early life stages of the sea urchin Evechinus chloroticus

<p>Metals are a common source of pollution in coastal waters, and have long been recognised as a major concern for many marine species, especially their early life stages. Although effects have been examined using standard toxicity assays, the impact of metals in more complex and realistic exposure regimes is still poorly known, in particular with regards to latent effects across multiple life stages and the interaction of multiple stressors. In this thesis, the effects of metals were investigated for multiple life stages of the endemic New Zealand sea urchin Evechinus chloroticus. Standard short-term bioassays were performed on the early life stage of E. chloroticus and also the endemic abalone Haliotis iris, for comparison. These assays evaluated the toxicity of three major pollutants (copper, lead and zinc) alone and in combination, on these species. Embryos of both species were highly vulnerable to copper (EC50s: 5.4 and 3.4 µg/L respectively for E. chloroticus and H. iris) and zinc (27.7 and 13.1 µg/L) but relatively tolerant to lead (52.2 and 775 µg/L), and there was no evidence of synergistic effects of metal mixtures. The latent effects of copper across two life stages in E. chloroticus, larval and juvenile, were investigated with laboratory experiments using realistic scenarios of low copper concentration, short pulses of exposure and examining exposure through dietary intake as well as waterborne exposure. Strong latent and carry-over effects were observed even at low concentration and short exposure time. For example, individuals exposed as larvae to 10.4 µg/L Cu for two days developed normally during the larval stage but had strongly impaired subsequent growth, with average body size decreasing by 24% in the 25 d following settlement. Moreover, juveniles previously exposed to copper as larvae were less resistant to a subsequent exposure, with up to four times higher mortality. Latent effects were especially important when copper was present in the diet rather than dissolved in water. For example, E. chloroticus larvae exposed to 2.3 µg/L Cu in water and fed with an algal diet cultured in the same concentration had a settlement success three times lower than those exposed only to waterborne copper. Furthermore, a short pulse exposure (4 days) to copper in the algal diet was generally more toxic than chronic exposure, showing that a short-lived bloom of contaminated phytoplankton may have a more severe impact on zooplankton than chronic pollution. Because metal discharge in coastal water is generally associated with freshwater (e.g. storm water or river plumes), the toxicity of copper was evaluated in both normal and low salinity seawater. Low salinity (24 ppt) increased copper toxicity in E. chloroticus larvae under chronic exposure to high levels (15 µg/L; 43% and 80% lower survival and normal development rate, respectively) but not under a single pulse exposure (4 days) to low concentration (5 µg/L). This highlights the importance of using realistic exposure in laboratory assays. Finally, the effect of copper on adult E. chloroticus and in particular on their fertilisation success was evaluated. Strong sublethal effects were observed after exposure to 50 µg/L Cu for two weeks including spawning impairment (especially in females) and elevated copper burden in gonads (25-times higher than control animals). However, the fertilisation success of successfully spawning males was not affected. The prevalence of local metal contamination was also measured at the mouth of local river plumes and in E. chloroticus gonads at sites expected to vary in likely exposure to pollution. Copper levels exceeding water quality criteria were found in two instances in coastal agricultural runoff (Makara stream). Other metals were within water quality cirteria in all samplings. Adult E. chloroticus had an elevated copper burden in gonads in an urban site compared to a control site (0.77 µg/g vs. 0.27 µg/g). In total, this research demonstrates the need for considering toxic effects across multiple life stages and using realistic exposure regimes (e.g. timing, concentration, multiple stressors) to better understand the likely impact of metal pollution on marine populations. It also provides the first measure of metal toxicity on early life stages of an endemic species of cultural and commercial importance in New Zealand.</p>

Internal Waves as a Source of Concentric Rings within Small River Plumes

This study is focused on concentric rings, which are regularly observed by remote sensing of small river plumes located in different regions worldwide. We report new aerial observations of these features obtained by quadcopters and supported by synchronous in situ measurements, which were collected during the recent field survey at the Bzyb river plume in the eastern part of the Black Sea. Joint analysis of remote sensing imagery and in situ data suggest that the observed concentric rings are surface manifestations of high-frequency internal waves generated in the vicinity of the river mouth. The obtained results demonstrate that the propagation of these waves does not induce offshore material transport within the plume induced by shear instability, which was hypothesized in a recent numerical modeling study of this process. We provide an explanation for the appearance of misleading material features in the numerical simulations discussed above. Finally, we discuss directions for future research of high-frequency internal waves generated in small river plumes.

On Using Lagrangian Drift Simulations to Aid Interpretation of in situ Monitoring Data

One key challenge of marine monitoring programs is to reasonably combine information from different in situ observations spread in space and time. In that context, we suggest the use of Lagrangian transport simulations extending both forward and backward in time to identify the movements of water bodies from the time they were observed to the time of their synopsis. We present examples of how synoptic maps of salinity generated by this method support the identification and tracing of river plumes in coastal regions. We also demonstrate how we can use synoptic maps to delineate different water masses in coastal margins. These examples involve quasi-continuous observations of salinity taken along ferry routes. A third application is the synchronization of measurements between fixed stations and nearby moving platforms. Both observational platforms often see the same water body, but at different times. We demonstrate how the measurements from a fixed platform can be synchronized to measurements from a moving platform by taking into account simulation-based time shifts.

Landscape biogeography and population structuring of a facultatively amphidromous galaxiid fish, Galaxias brevipinnis

Processes responsible for population structuring across spatial and temporal scales represent key components in understanding speciation and evolution. We use a hierarchical approach to investigate the degree and mechanisms of structuring in landlocked and diadromous populations of the facultatively amphidromous fish Galaxias brevipinnis across various temporal and spatial scales in southern New Zealand. To determine long-term structuring, multiple lakes and coastal sites were compared genetically. Short-term structuring was assessed using otolith microchemistry for a subset of sites, and behavioural mechanisms driving population structuring were assessed via larval distributions. Genetic data show that lakes foster divergence of lake-developing populations from each other and from coastal stream populations, whereas there is relatively little structuring within coast or lake populations. However, otolith analyses indicate that on a shorter time scale, most larvae do not disperse, i.e. recruitment is local. Thus, lake and coastal populations show a distinct meta-population structure based on catchment, in contrast to the prevailing assumption of widespread dispersal, with implications for management. Most larvae were distributed in river plumes, suggesting that a simple larval behavioural mechanism, e.g. positive rheotaxis, may result in larval retention within catchments and lakes. However, not all larvae were retained in plumes, creating opportunities for genetic exchange within-lake or among coastal sites. Genetic divergence of lake populations as a consequence of landscape and behaviour provides an insight into the potential of G. brevipinnis to diversify and speciate, when landscape and circumstances align, and also has implications for the management of this and other facultatively amphidromous species.

Are Periodic Oscillations of Sea Surface Height Inherent to River Plumes?

We analyze high temporal resolution (10 min) sea surface height (SSH) data records from pressure sensors deployed on the inner shelf within the plumes of the Mzymta and the Vulan, two small-to-medium size rivers in the Russian sector of the Black Sea coast. The SSH was recorded in parallel with meteorological parameters, averaged over the same 10 min intervals. While the low-frequency changes of the SSH at periods of 6 h or longer accounted for about 90% of the total variability and were strongly correlated with the wind, superimposed on them there always existed higher frequency oscillations whose characteristic periods ranged from 70 to 230 min, and the amplitudes spanned between 1 and 19 cm, not correlated with either the wind stress or atmospheric pressure. Furthermore, the amplitudes and the periods of these high-frequency oscillations appeared to be proportional to the horizontal scale of the river plume, as well as to each other. A very simple semi-analytical model focused on the interplay between the plume’s mass and the momentum budgets demonstrated that periodic oscillations of the sea surface height should be inherent to river plumes, and also helped to explain the abovementioned dependencies.