Assemblage structure and trophodynamics of mesopelagic fishes in the Benguela and Canary Current Upwelling Systems

Abstract

Mesopelagic fishes, populating habitats in the twilight zone between ca. 200 -1000 m depth, play a vital role in the marine food web because of their large biomass and their function in the ocean's biological carbon pump; many species perform diel vertical migrations and feed near the surface at night and then migrate to deeper layers during the day to avoid predation. As a result, mesopelagic fishes actively transport organic carbon as by-product of respiration and excretion from the epipelagic to the twilight zone. Although mesopelagic fishes are a considerably large proportion of the biomass of marine food webs and are prey to many commercially important species, their ecology is still understudied, especially in highly productive areas such as Eastern Boundary Currents. The present thesis sheds light on the abundances, community composition, and trophic ecology of mesopelagic fishes and the environmental factors that affect their assemblages in the two Eastern Boundary Upwelling Systems of the Atlantic. Chapter I examines the community composition of mesopelagic fishes off the coast of northwest Africa in the Senegalese-Mauritanian sub-region of the Canary Current, while Chapters II and III compare the assemblage and trophic structure of mesopelagic fishes in the northern (nBUS) and southern (sBUS) Benguela Upwelling Systems. We found that mesopelagic fish assemblages are largely driven by water mass properties in these upwelling systems and that oxygen was one of the main environmental factors that affected community composition (Chapters I and II). Warm and poorly oxygenated South Atlantic Central Water (SACW) is the dominant water mass of both the Mauritanian Upwelling Region (MUR) and the northern Benguela. In both of these regions, communities were dominated by ‘tropical’ warm water species. In the nBUS the examples of warm water species were myctophids Notoscopelus resplendens and Diaphus taaningi whereas in the MUR, dominant warm water taxa were Diaphus vanhoeffeni (Myctophidae) and Polyipnus polli (Sternoptychidae). In the Mauritanian sub-region, there was a pronounced oxygen minimum zone (OMZ) that spanned from about 40 m to over 600 meters, while in the nBUS, the OMZ extended off the shelf but did not span a large portion of the water column. These oxygen conditions were also represented in the species composition in both regions; 24-62% of the mesopelagic fish assemblages of the MUR were made up of the bristlemouth Cyclothone (Gonostomatidae), which is known to be tolerant of low oxygenated waters and resides in the OMZ. In contrast, in the nBUS’ OMZ, species richness was very low and was dominated by the low-oxygen tolerant lanternfish Diaphus dumerilii (Myctophidae). In the northern sampling region of the MUR there was mixing with the cooler North Atlantic Central Water which resulted in a mixture of tropical an temperate species like the myctophid Benthosema glaciale. The southern Benguela communities were characterized by the influence of several water masses that led to a range overlap of species ecologically divergent in their biogeography. The sBUS was dominated by Eastern South Atlantic Central Water but also influenced by the Agulhas Current with species from the Indian Ocean. A common cold-water species was Lampanyctodes hectoris (Myctophidae) whereas a warm-water species typical of the Indian Ocean was Diaphus diadematus. Despite the mesopelagic zone being generally characterized as homogeneous habitats and inhabited by species with large biogeographical ranges, Chapters I and II show considerable changes in mesopelagic communities in relation to hydrographic conditions in productive upwelling systems. When comparing communities between the nBUS and sBUS, we also found that there were differences between assemblages on the shelf and offshore. In the sBUS, the shelf region had very low richness but high abundance of Maurolicus walvisensis (Sternoptychidae). The shelf of the nBUS not only had low richness as previously mentioned, but also a low abundance of fishes overall. This may be a result of the OMZ or the high abundance of jellyfish found on the shelf. In contrast to shelf communities, offshore assemblages in both subsystems had both high richness and abundance of fishes from a number of mesopelagic families including Myctophidae, Sternoptychidae, Stomiidae, Gonostomatidae, and Bathylagidae. Community composition can have an impact on the food web in a system since differing taxa have various feeding patterns. Our results from Chapter II show that vertically-migrating species of myctophids are dominant zooplanktivores in the nBUS and sBUS mesopelagic assemblages. Piscivorous stomiids were also found in both subsystems but at much lower abundances. In Chapter III, we compared the trophic niches of migrating (feeding in the epipelagic zone) and non-migrating (feeding in the mesopelagic zone) zooplantivores and piscivores using stable isotopes. We found a high variation in the δ15N within many zooplanktivorous species, suggesting that zooplanktivores, especially myctophids, occupy a rather wide range of trophic positions. It has been reported that copepods, an important component of their prey, span multiple trophic levels in the Benguela which would explain the wide trophic niches within groups further up in the food chain, including piscivorous mesopelagic fishes which also occupied large trophic niches. The trophic enrichment factors between the baseline and zooplanktivores ranged from 3.8 to 7.5‰, while the enrichment factor between zooplanktivores and piscivores was between -0.8 and 1.6‰. This decreasing enrichment with increasing trophic level, while in itself not unusual, was higher than expected. One possible reason might be that some species would need to be categorized differently as they prey at least partially on fishes, whereas they were previously believed feed merely on zooplankton. The high abundance of mesopelagic fishes in the Benguela, especially of vertically migrating species, make these fishes important components in the food web in the nBUS and sBUS and our results suggests that their community composition will have implications on the trophic transfer efficiency within each of the systems.