Sub-surface hotspots in shallow seas: fine-scale limited locations of top predator foraging habitat indicated by tidal mixing and sub-surface chlorophyll

Abstract

The foraging habitats of 7 species of marine apex predators were observed simultaneously in a shallow sea, with continuous measurements taken of the detailed bio-physical water column characteristics to determine habitat preferences. We found the occurrence of small-scale 'hotspots', where 50% of all animals were actively foraging in less than 5% of the 1000 km of transects surveyed. By investigating a contrasting range of foraging strategies across a variety of fisheating seabirds and marine mammals, we determined which habitat characteristics were consistently important across species. A static habitat variable, tidal stratification, log 10 (h/U 3 ) (h = water depth, U = tidal current amplitude), was found to be the best indicator of the probability of presence and abundance of individual species. All 7 mobile top-predators preferentially foraged within habitats with small-scale (2 to 10 km) patches having (1) high concentrations of chlorophyll in the sub-surface chlorophyll maximum (CHL max ) and (2) high variance in bottom topography, with different species preferring to forage in different locations within these habitats. Patchiness of CHL max was not associated with the locations of strong horizontal temperature gradients (fronts) or high surface chlorophyll values, but instead may be related to areas of high sub-surface primary production due to locally increased vertical mixing. These small-scale areas represent a newly identified class of spatially important location that may play a critical role within the trophic coupling of shallow seas. Such subsurface hotspots may represent the limited locations where the majority of predator-prey interactions occur, despite making up only a small percentage of the marine environment.KEY WORDS: Biological hotspots · Foraging habitats · Marine top predators · Predator-prey interactions · Shallow sea · Sub-surface chlorophyll maximum · Tidal mixing · Topography 408: 207-226, 2010 Review studies on the upwelling systems of the Pacific Ocean identified spatially and temporally predictable 'biological hotspots' with distinct surface signatures (Spear et al. 2001, Bakun 2006, Ballance et al. 2006, Sydeman et al. 2006. At this large scale, the mechanistic evidence behind the creation of hotspots for marine predator foraging points directly to both topographical features (Genin 2004, Yen et al. 2004 and primary productivity (Ware & Thomson 2005, Bost et al. 2009). The general conclusion of many largescale studies is that seabirds and marine mammals are found preferentially foraging within different types of frontal region: areas of intersection between different water mass types where there are steep surface (horizontal) gradients in water density. Biological reasoning links foraging to frontal locations via the elevated levels of primary production and aggregation of planktonic organisms found at fronts (Pingree et al. 1975, Franks & Chen 1996, Durazo et al. 1998, Russell et al. 1999, Lough and Manning 2001. However, some of the studies mentioned above stress...

Mesoscale fronts as foraging habitats: composite front mapping reveals oceanographic drivers of habitat use for a pelagic seabird

“…The reasons for these differences are not clear but are likely to be related to differences in regional oceanography. Small-scale, superficial and ephemeral thermal fronts develop frequently in the Celtic Sea through tidal effects and cycles of stratification and mixing [30] but are not always associated with chl-a enrichment [28,66]. By contrast, the Benguela is a major upwelling zone, in which upwelling filaments, eddies and strong vertically structured fronts manifest.…”

“…Sea-surface temperature (SST) and chlorophyll-a (chl-a) imagery are most widely used [12,32], but it is questionable whether these metrics are appropriate for defining foraging habitat, particularly for piscivores [33]. Indeed, the use of chl-a imagery in shallow shelf seas could be misleading, as sub-surface chlorophyll maxima in stratified areas can present more attractive foraging opportunities than mixed waters with elevated surface chl-a [28]. By contrast, sub-surface processes occurring along thermal fronts are known to increase prey accessibility for diving predators.…”

“…However, a key constraint is the lack of data describing oceanographic processes and pelagic prey distributions at matching spatio-temporal scales. Although in situ studies have yielded valuable insights into the fine-scale mechanisms underlying animal-oceanography interactions [28][29][30][31], this Eulerian approach cannot provide information on behaviour throughout a foraging bout, limiting our understanding of broader-scale oceanographic influence. Remotely sensed data can supplement bio-logging, identifying physical conditions that drive habitat selection in virtual realtime.…”

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

“…Some studies have shown the presence of methanogenic microorganisms in these particles and have suggested that methanogenic activity originates in the digestive tracts of zooplankton and in fecal pellets. Below the ice pack, subsurface peaks of phytoplankton biomass persist, called the subsurface chlorophyll maxima (Arrigo et al, 2011), which has recently been described as an important site for predator-prey interactions (Scott et al, 2010). Thus, the existence of an abundant and active zooplankton community (including protozooplankton), and therefore pellet production (Hirche and Mumm, 1992;Morata and Seuthe, 2014), may also support anaerobic methanogenesis, playing a critical role in trophic coupling.…”

Environmental control of the breeding success of rhinoceros auklets at Triangle Island, British Columbia

“…Alternately, local foraging hotspots for a suite of marine predators sometimes exist where a highly disproportionate amount of trophic energy exchange is focused in very small areas that feature high primary production (e.g. Scott et al 2010). Given the extreme interannual variation in the productivity of the marine environment around Triangle Island, and its documented effects on consumers in this system -including the seabirds at Triangle Island (Mackas et al 2007) -we set out to test the hypotheses that: (1) interannual variation in local marine primary productivity, particularly in spring (as crudely indicated by satellite-derived phytoplankton chlorophyll concentrations) is a critical factor driving interannual variation in the breeding success of the rhinoceros auklet, a generalist forager in this system (Davies et al 2009); (2) that the effect would be due at least in part to its effects on the recruitment of a critical prey species, Pacific sandlance Ammodytes hexapterus, and thus on its availability to the auklets, as suggested previously by Bertram et al (1991) and Hedd et al (2006);and (3) that relationships between satellite-derived chlorophyll concentration and seabird success would be applicable to other consumers in the system.…”