Sahara Coastal - Ecology

As in most Eastern boundary current upwelling ecosystems, upwelling related productivity is a function of latitude as well as upwelling strength. Productivity of the system follows the upwelling cycle. Since new nutrient input and subsequent primary production (and the variability of these) can significantly affect upper trophic level productivity and community structure through energy moving up the food web (tropic levels), bottom-up control can be expected. However, trophic control within the ecosystem is also the result of wasp-waist influences of small pelagic fish, as well as top-down predatory or fishery controls. Indeed, small pelagic species which constitute the bulk of the biomass in this upwelling area may play a major functional role in this ecosystem. Small plankton-feeding pelagic species occupy a crucial mid-trophic position in the food web. These species support important fisheries, mainly for fish meal and are important food sources for larger fish, seabirds, and marine mammals.
A number of critical physical processes are likely to govern much of the variability in small pelagic populations in this upwelling area:

• Alongshore advection
• Timing, duration, and intensity of coastal upwelling
• Vertical mixing events
• Mesoscale eddies, jets, and meanders

Variability in small pelagic fish due to fishery removals (especially if removals of those lower trophic level species reach risky limits that impair bottom-up production transfer) or to recruitment variability may introduce alteration in energy and material flow through food webs. In the decade extending from 1990 to 2000, large variability of the population of small pelagic fishes has been observed in the region, with migration of species and decreases in fish stocks. This variability seems to be highly correlated with changes in the physical properties of the coastal waters due to anomalies in the upwelling patterns. Thus small pelagic fish variability is expected to introduce a "wasp-waist" control of the trophic dynamics in this upwelling ecosystem.
There is also evidence of top-down control in the Moroccan ecosystem. The area was uncontrollably exploitated during the 1960s, which drastically reduced stocks of sparids and seems to have facilitated the outburst of short lived cephalopods after the late 1960s. Heavy fishing during the 1960s removed cephalopod predators (Caddy, 1983, Caddy and Rodhouse, 1998), while discards by the trawl fishery are suspected to have enhanced the abundance of some scavenger species that constitute cephalopod prey (Balguerias et al., 2000).
Understanding the relative importance of each type of control in the Moroccan food web is a focus of current ecological studies in the region.