Response of small sharks to nonlinear internal waves

Abstract

Plankton and nekton may respond passively or actively to large-amplitude, nonlinear internal waves (NLIW), with periods and wavelengths on the order of minutes and hundreds of meters, and the NLIW can cause direct or indirect changes in distribution. NLIW are ubiquitous in the coastal ocean, but understanding the influence of NLIW on organism response and distribution is challenging, because of NLIW unpredictability, short temporal and spatial scales, and the difficulty in resolving the biological response. Measurements of currents, temperature, and shark acoustic traces in Massachusetts Bay were used to evaluate the short-term response of individuals as well as the mean effects on the distribution of an aggregation of small sharks, Squalus acanthias. In two NLIW events, we detected 527 and 3240 shark traces. Individuals moved up and down in response to the currents associated with the sinking and rising of the thermocline. However, mean distribution deepened during one of the events, suggesting that organisms did not merely move in concert with the thermocline oscillation, but that sharks instead might have responded actively. Measurements of vertical currents and shark's depth change during one of the NLIW events indicate that with downward currents (sinking of the thermocline), the sharks tend to react passively. However, in response to the fastest upward currents, sharks appeared to swim down, supporting an active response in the rising phase of the wave. NLIW and other high-frequency processes can have a profound influence on the distribution of pelagic organisms, yet their ecological consequences remain largely unaccounted for.

Internal seiches as drivers of fish depth use in lakes

“…In summary, our results show that internal seiche events in lakes can drive the vertical positioning of fish. This observation builds upon other studies that also found fish in different thermally stratified systems also respond to internals waves (Levy et al 1991;Pineda et al 2020;Flood et al 2021b). The compression of walleye into oxygen-rich and warm epilimnetic waters will increase their metabolic demand, therefore requiring an increase in prey biomass for sustenance.…”

Internal solitary waves on the NW African shelf: A heuristic approach to localize diapycnal mixing hotspots

“…Estimation of turbulent mixing on the continental shelf is important for understanding the structure and dynamics of marine ecosystems, nutrient cycling, primary production and dispersion of pollutants (Nash et al, 2004;Pineda et al, 2020;Rippeth et al, 2005;Schafstall et al, 2010). Diapycnal mixing in the ocean is mostly driven by intermittent patches of small-scale turbulence (Müller and Briscoe, 2000).…”

“…Physical processes, such as IWs, can be observed via spatial and temporal variations in zooplankton layers, whose acoustic responses vary, according to their observed frequency and which are visible with low acquisition thresholds (no more than -80 dB based on the assumption of a backscatter around -70/-75 dB for the zooplankton layers) (Bertrand et al, 2010;Pineda et al, 2020). Furthermore, Sandstrom et al (1989) indicated that acoustic backscatter levels in turbulent patches approach -50 dB at 200 kHz, providing a rapid and convenient survey tool for estimating mixing levels induced by IWs.…”

Concentration and condition of American lobster postlarvae in small‐scale convergences

“…Small‐scale pelagic habitat heterogeneities are associated with physical convergent and divergent processes, such as estuarine fronts, with surface across‐front hydrographic gradients (Govoni & Grimes, 1992; O'Donnell et al, 1998) and internal waves (Alpers, 1985), with generally no surface hydrographic variability (but see Pineda et al, 2020, for highly nonlinear internal waves). Mixing can modulate the erosion of small‐scale heterogeneities (O'Donnell, 1990), but upper layer mixing by strong winds (>10 m/s) and surface waves can also be associated with convergent and divergent Langmuir circulation (Kukulka et al, 2009).…”

“…Small-scale pelagic habitat heterogeneities are associated with physical convergent and divergent processes, such as estuarine fronts, with surface across-front hydrographic gradients (Govoni & Grimes, 1992;O'Donnell et al, 1998) and internal waves (Alpers, 1985), with generally no surface hydrographic variability (but see Pineda et al, 2020, for highly nonlinear internal waves).…”