Remote sensing of zooplankton swarms

Abstract

Zooplankton provide the key link between primary production and higher levels of the marine food web and they play an important role in mediating carbon sequestration in the ocean. All commercially harvested fish species depend on zooplankton populations. However, spatio-temporal distributions of zooplankton are notoriously difficult to quantify from ships. We know that zooplankton can form large aggregations that visibly change the color of the sea, but the scale and mechanisms producing these features are poorly known. Here we show that large surface patches (>1000 km2) of the red colored copepod Calanus finmarchicus can be identified from satellite observations of ocean color. Such observations provide the most comprehensive view of the distribution of a zooplankton species to date, and alter our understanding of the behavior of this key zooplankton species. Moreover, our findings suggest that high concentrations of astaxanthin-rich zooplankton can degrade the performance of standard blue-green reflectance ratio algorithms in operational use for retrieving chlorophyll concentrations from ocean color remote sensing.

In a comfort zone and beyond—Ecological plasticity of key marine mediators

“…Additionally, Basedow et al. (2019) observed singular individuals of C. glacialis offshore from the northern Norwegian coast, which implies that the fjord and shelf populations of C. glacialis can be transported far offshore over the shelf edge and thus can be mixed with C. finmarchicus even within the core of its northward flowing Atlantic water current.…”

“…In this case, the vertical position of copepods could indeed play a role, as C. finmarchicus most likely persists in intermediate water layers in the fjord, where Atlantic water is advected to the Hornsund (Promińska et al., 2018). This deeper occurrence may explain their reduced need for protection against UVR irradiance in the fjord in contrast to the increased photoprotection needed in the offshore Atlantic waters, where C. finmarchicus tends to concentrate in the upper few meters (Basedow et al., 2019; Trudnowska et al., 2015).…”

The hidden influence of large particles on ocean colour

“…Using Mie theory, we have clearly identified the physical basis through which large, coloured particles can affect bulk IOPs, particularly absorption, and therefore impact on remote sensing reflectance signals. The potential impact of one species of zooplankton on ocean colour signals has previously been discussed 23 . The results presented here generalise the situation to include the whole variety of mm-class colored particles found in the ocean, including other species of copepods, krill, fish eggs, fish larvae, and non-living suspended particulate material.…”

“…The optical properties of the ocean are traditionally considered to be determined by the effect of dissolved organic materials and numerically abundant phytoplankton, detritus and other sediment and colloidal particles 2 , 20 , 21 . However, recent evidence suggests that larger but less abundant particles such as mm-sized particles (including zooplankton) may also be present in high concentrations near the surface 22 , so much so that they can influence ocean colour signals 23 – 27 . However, a discrepancy between remote sensing signals and in situ inherent optical property (IOP) measurements remains 23 .…”

“…However, recent evidence suggests that larger but less abundant particles such as mm-sized particles (including zooplankton) may also be present in high concentrations near the surface 22 , so much so that they can influence ocean colour signals 23 – 27 . However, a discrepancy between remote sensing signals and in situ inherent optical property (IOP) measurements remains 23 . It appeared that remote sensing signals could have be influenced by large particle absorption while IOP measurements may not have been sensitive to this class of particles.…”