A marine zooplankton community vertically structured by light across diel to interannual timescales

Abstract

The predation risk of many aquatic taxa is dominated by visually searching predators, commonly a function of ambient light. Several studies propose that changes in visual predation will become a major climate-change impact on polar marine ecosystems. The High Arctic experiences extreme seasonality in the light environment, from 24 h light to 24 h darkness, and therefore provides a natural laboratory for studying light and predation risk over diel to seasonal timescales. Here, we show that zooplankton (observed using acoustics) in an Arctic fjord position themselves vertically in relation to light. A single isolume (depth-varying line of constant light intensity, the value of which is set at the lower limit of photobehaviour reponses of Calanus spp. and krill) forms a ceiling on zooplankton distribution. The vertical distribution is structured by light across timescales, from the deepening of zooplankton populations at midday as the sun rises in spring, to the depth to which zooplankton ascend to feed during diel vertical migration. These results suggest that zooplankton might already follow a foraging strategy that will keep visual predation risk roughly constant under changing light conditions, such as those caused by the reduction of sea ice, but likely with energetic costs such as lost feeding opportunities as a result of altered habitat use.

Keywords: Arctic; isolume; migration; predation; zooplankton.

Figure 1.

The seasonal and diel response of the meso-zooplankton community (observed using ADCP data (mean volume backscattering strength, S_v (dB))) to ambient light. All data acquired from 2013 to 2014. (a) Modelled incoming irradiance at midday (dot–dashed) and midnight (solid) at the sea surface; (b) chlorophyll-a concentration, as measured by a fluorometer at 38 m; (c–h) acoustic backscatter measured using two ADCPs at approximately 100 m. Dotted black lines show the depth of isolumes within the range of copepod and krill lower limit photobehaviour thresholds (10⁻⁸–10⁻⁶ µmol photons m⁻² s⁻¹ [26,27]), while the solid black line is the midpoint of this range (10⁻⁷ µmol photons m⁻² s⁻¹). (c,d) The full seasonal cycle, with backscatter data at local midday (c) and midnight (d). (e–h) Diel behaviour, with depth extracted for 48 h periods centred on (e) 21 December (winter solstice); (f) 20 March (spring equinox); (g) 15 June (near summer solstice, peak of spring bloom); (h) 6 September (closest data available to the autumn equinox). Grey lines on (c–h) indicate the depth of the centre of mass of backscatter. Vertical dashed lines on (c,d) correspond to dates of data extraction for (e–h).

Figure 2.

Interannual variation in the seasonal response to light. (a,c,e) Chlorophyll time series; (b,d,f) ADCP data (mean volume backscattering strength, S_v (dB)) at midnight from the top approximately 100 m, with isolumes as per figure 1. Data are shown from 3 years in Kongsfjorden. Chl_max indicates the maximum Chl-a concentration each year as a way of comparing spring bloom (and therefore shading) magnitude, with plots ordered by Chl-a magnitude.