Leveraging eDNA metabarcoding to characterize nearshore fish communities in Southeast Alaska: Do habitat and tide matter?

Nearshore marine habitats are critical for a variety of commercially important fish species, but assessing fish communities in these habitats is costly and time-intensive. Here, we leverage eDNA metabarcoding to characterize nearshore fish communities near Juneau, Alaska, USA, a high-latitude environment with large tidal swings, strong currents, and significant freshwater input. We investigated whether species richness and community composition differed across three habitat types (sand beaches, eelgrass beds, and rocky shorelines) and between high and low tides. Additionally, we tested whether replication of field samples and PCR reactions influenced either species richness or composition. We amplified a 12S mitochondrial locus in our samples and identified 188 fish amplicon sequence variants (ASVs), corresponding to 21 unique taxa, with approximately half of these resolved to single species. Species richness and composition inferred from eDNA differed substantially among habitats, with rock habitats containing fewer taxa and fewer overall detections than sand and eelgrass habitats. The effect of tide was more subtle and suggested a habitat-tide interaction, with differences in taxa between tides largely isolated to sand habitats. Power analyses indicated that additional field sampling is useful to detect subtle changes in species richness such as those due to tide. PCR replicates typically identified a small number of additional taxa. The most notable result from our study was that shore morphology appeared to substantially influence community structure. Rocky shorelines sloped quickly into deep water, while sand and eelgrass habitats descended much more gradually. We hypothesize that differences in taxa observed among habitats were largely due to lack of mixing between bottom and surface water, providing further evidence that eDNA transport is minimal and that many marine eDNA detections are derived from highly localized sampling locations. We suggest that future studies could explore the extent to which habitat and nearshore physical processes influence eDNA detections.

Large-scale scour in response to tidal dominance in estuaries

<p>Channel beds in estuaries and deltas often exhibit a local depth maximum at a location close to the coast. There are two known causes of large-scale (i.e. >10 river widths along-channel) channel bed scours: width constriction and draw down during river discharge extremes, both creating a local flow acceleration. Here, we systematically investigate a potential third mechanism. We study the effect of tidal dominance on the equilibrium channel bed in estuaries with a 1D-morphodynamic model. In estuaries, a morphodynamic equibrium is reached when the net (seaward) transport matches the upstream supply along the entire reach. The residual (river) current and river-tide interactions create seaward transport. Herein, river-tide interactions represent the seaward advection of tide-induced suspended sediment by the river flow. Tidal asymmetry typically creates landward transport. The main reason for scour formation is the amplification of tidal flow through funnelling of tidal energy. Only for a scouring profile the drop in river induced current magnitude reduces the river-tide interaction term, so that the net sediment transport matches the upstream sediment transport. When tidal influence is relatively large, and when channel convergence is strong, a equilibrium is only obtained with a scouring profile. We propose a predictor dependent on the width convergence, quantified as S<sub>B</sub>, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (q<sub>tide</sub>/q<sub>river</sub>). Scours develop if (q<sub>tide</sub>/q<sub>river</sub>)/S<sub>B</sub> exceeds 0.3. These results are independent of scale and allow the prediction of scour in estuaries under future changes.</p>

The time-varying characteristics in tidal duration asymmetry

<p>Tides always behaves different rising and falling durations, which can mostly attribute to the shallow-water effect and interactions among tidal constituents. The duration asymmetry may lead to an inequality in flood/ebb tidal current magnitudes, affecting the net sediment transport. Tidal duration asymmetry has time-dependent characteristics. We deducted a general framework for identifying the time-variability in tidal duration asymmetry. The application to the global tides showed that the fortnightly variability in tidal asymmetry is universal and that duration asymmetry can be stronger during neap tide than during spring tide. Then the framework is applied to the tides in the Changjiang Estuary. Prominent seasonal variation in tidal asymmetry is revealed, mainly relate to the river-tide interaction. Application to the tides in the Yangshan Harbor sea area revealed that the local-scale tidal asymmetry can be changed strongly by a large coastal engineering.</p>

The upper atmospheric responses to tidal and planetary waves

<p>Tidal and planetary waves (PWs) in the mesosphere and lower thermosphere region could have significant impact on the upper thermosphere/ionosphere system through direct propagations, E region wind dynamo, and the change of residual circulations. We would like to show some results from BeiDou and COSMIC observations, as well as TIME-GCM simulations, to illustrate the lower/upper atmospheric couplings through different mechanisms. Generally, the spatial structures of the ionospheric responses to planetary waves agree with the ionospheric fountain effect, which indicates the important roles of equatorial wind dynamos in transmitting planetary wave signals to the ionosphere. The TIME-GCM simulations show that the zonal and meridional components of the planetary waves could result in evident vertical ion drift perturbations, while the net ionospheric effect is related to both their latitudinal structures and phases. The simulations also show that the change of tidal amplitudes and secondary PWs generated by PW-tide interaction are also important to the ionospheric variabilities. Besides, the couplings through PW-induced residual circulations are exhibited by both model simulations and TEC observations from BeiDou satellite system.</p>

TIDE-TSUNAMI INTERACTION IN A HIGHLY ENERGETIC CHANNEL. A CASE STUDY.

Tsunami–tide interaction can be assessed using different approaches with increasing levels of complexity. The simplest is to compute the sea level through a linear superposition of the tide and the tsunami computed independently (composite model). Recent studies have found that composite models provide inaccurate results in shallow waters (e.g. Kowalik et al, 2010). A more realistic analysis is achieved by computing the tsunami and the tide together (full model). This approach is appropriate where nonlinear effects may be important due to strong tides or shallow bathymetries. This work is intended to improve the physical understanding of tide-tsunami interaction in Canal Chacao, a highly energetic channel sited in Chile. This channel is dominated by currents of up to 6 [m/s] during spring tide and is located in a region prone to tsunamis. The fundamental question is to assess under which conditions tides and tsunamis can be linearly superposed and in which they interact nonlinearly, thus enhancing or reducing the surface elevation and associated currents.

EFFECTS OF TIDE AND WAVE DIRECTIONALITY ON LOCALIZED TSUNAMI-INDUCED CURRENTS IN PORT AND HARBORS

Here, we present the results of a numerical modeling study to investigate how the maxima of the tsunami-induced currents vary due to dynamic effects of tides and wave directivity. A sensitivity analyses was conducted in three harbors by coupling the tsunami with the tide signal at twelve different tide levels. We find that tsunami-tide interaction can change the maximum current speed experienced in a harbor by up to 25% for the events and harbors studied, and that this effect is highly site-specific. To evaluate the effect of wave directionality on maximum currents, three earthquakes with different magnitudes were devised along the Pacific, which were also tuned to create the same maximum near-harbor amplitude. Our analysis also shows that, for the harbor and sources examined, the effect of offshore directionality and tsunami frequency content has a very weak effect on the maximum currents experienced in the harbor. The much more important dependency on maximum currents is on the near-harbor amplitude of the wave, indicating that currents in a harbor from a tsunami generated by a large far-field earthquake may be reasonably well predicted with only information about the predicted local tsunami amplitude. This study was motivated by the hope of constructing a basis for understanding the dynamic effects of tides and wave directivity on current-based tsunami hazards in a coastal zone by the application of numerical simulation tools for hazard mapping purposes. The consideration of these aspects is crucial and yet challenging in the modeling of tsunami currents.