Climate influence on deep sea populations

Abstract

Dynamics of biological processes on the deep-sea floor are traditionally thought to be controlled by vertical sinking of particles from the euphotic zone at a seasonal scale. However, little is known about the influence of lateral particle transport from continental margins to deep-sea ecosystems. To address this question, we report here how the formation of dense shelf waters and their subsequent downslope cascade, a climate induced phenomenon, affects the population of the deep-sea shrimp Aristeus antennatus. We found evidence that strong currents associated with intense cascading events correlates with the disappearance of this species from its fishing grounds, producing a temporary fishery collapse. Despite this initial negative effect, landings increase between 3 and 5 years after these major events, preceded by an increase of juveniles. The transport of particulate organic matter associated with cascading appears to enhance the recruitment of this deep-sea living resource, apparently mitigating the general trend of overexploitation. Because cascade of dense water from continental shelves is a global phenomenon, we anticipate that its influence on deep-sea ecosystems and fisheries worldwide should be larger than previously thought.

Figure 1. Map of the study area.

Bathymetric map of the northwestern Mediterranean showing the location of the fishing harbors considered in this study (blue ships). Landings from Palma de Mallorca fishing harbor (purple ship) are included as supporting information (Figure S5). Moorings in the Cap de Creus Canyon (red circle) and in the basin (green square), and the Wavescan buoy deployed at 1200 m water depth off Palamós (black triangle) are also shown. Pale blue arrows indicate the pathway of the dense shelf water cascading mechanism extending from the Gulf of Lions along and across the continental slope, and the faded pink area represents the region affected by the thermo-haline and turbidity anomaly observed in the Western Mediterranean Deep Water after the 1999 and 2005 major cascading events. Both phenomena are inferred from published data –.

Figure 2. Mooring time series and daily catches.

(A–C) Current speed (A), temperature (B) and suspended sediment concentration (SSC) (C), recorded in the Cap de Creus Canyon at 500 m depth (placed at 5 m above bottom), before, during and after the cascading event of winter 2005. (D–G) Daily landings of Aristeus antennatus at the studied harbors are plotted as red bar charts, ordered from northeast (Roses) to southwest (Arenys de Mar). (H) Downward mass fluxes recorded at 2100 m depth by the sediment trap moored in the basin at 2350 m depth (i.e. 250 m above bottom) are illustrated by the orange bar chart. See locations in Figure 1.

Figure 3. Intra-annual variability of daily catches.

(A and B) Daily landings of Aristeus antennatus since 2001 at Palamós (A) and Blanes (B), the most representative studied harbors. Captures show a marked pattern of intra-annual variability with higher landings in spring-summer and lower landings in autumn, which has been attributed to reproductive displacements of the population . The pale yellow band shows the period affected by the temporary fishery collapse caused by the winter 2005 dense shelf water cascading event, disrupting the intra-annual variability of precedent years.

Figure 4. Temporal evolution of annual landings.

(A–D) Available annual landings of Aristeus antennatus at the studied harbors since 1977. Green dashed lines indicate years when strong cascading events occurred. A similar temporal evolution of landings can be observed in all harbors, with an increasing trend and a peak of captures between 3 and 5 years after the cascading events (pale yellow bands).

Figure 5. Temporal evolution in population structure.

(A–D) Estimation of the abundance of small (continuous red line) and large (dotted blue line) individuals of Aristeus antennatus derived from the annual landings in the studied harbors. Green dashed lines indicate the years when major cascading events occurred. The temporal evolution of the population structure of Aristeus antennatus in all harbors was linked to those cascading events. The number of small individuals increased two and three years after the event, prior the increase in the number of large individuals, indicating that cascading events enhance the recruitment process of this species.