Distribution Patterns of Oceanic Micronekton at Seamounts And
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Distribution patterns of oceanic micronekton at seamounts and hydrographic fronts of the subtropical Atlantic Ocean Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Rabea Diekmann Kiel 2004 Referent/in: Prof: Dr. D. Schnack Korreferent/in Tag der mündlichen Prüfung: Zum Druck genehmigt: Kiel, Der Dekan Contents Chapter I General Introduction 5 Chapter II Early life stages of cephalopods in the Sargasso Sea: 16 Distribution and diversity relative to hydrographic conditions II. 1 Introduction 17 II. 2 Material and Methods 18 II. 3 Results 20 II. 4 Discussion 27 Chapter III Species composition and distribution patterns of early life 31 stages of cephalopods at Great Meteor Seamount (subtropical NE Atlantic) III. 1 Introduction 32 III. 2 Material and Methods 33 III. 3 Results 37 III. 4 Discussion 45 Chapter IV Fish larvae and cephalopod paralarvae assemblage structures at 49 Great Meteor Seamount – a multivariate approach – IV. 1 Introduction 51 IV. 2 Material and Methods 53 IV. 3 Results 59 IV. 4 Discussion 74 Chapter V Vertical distribution and migration behaviour of cephalopods: 81 Implications on distribution patterns around seamounts V. 1 Introduction 82 V. 2 Material and Methods 83 V. 3 Results 87 V. 4 Discussion 97 Chapter VI General Discussion 102 Summary 112 Zusammenfassung 116 References 120 Appendix: Early life and juvenile cephalopods around seamounts of the I – XLIII subtropical eastern North Atlantic: Illustrations and a key for their identification Chapter I General Introduction The pelagic environment of open oceans seems to be the most monotonous habitat on our planet. It is generally characterised by low productivity and a weak horizontal physical structuring. In the vertical plain variability is stronger in the upper parts of the water column, especially in the pycnocline, but small in the meso- and bathypelagic layers. Small physical discontinuities lead to only slight ecological differences at small spatial scales, and few obvious ecological niches exist. At biogeographic scales, Longhurst (1998) distinguished primary (biomes) and secondary hierarchical areas (provinces) of the upper ocean for which unique ecological characteristics may be predicted. His definition of boundaries between compartments is mainly based on observations of primary productivity in the world’s oceans, that had become available in the past decades by remote sensing, imaging world-wide sea-surface chlorophyll concentrations. The boundaries between biomes, which stretch around the globe, are mainly congruent with major climatic zones. At this large scale typical latitudinal diversity gradients with high diversity in tropical seas and low diversity in polar regions have been postulated (e.g. Rex et al., 1993; Stehli et al., 1969). This latitudinal decline in species richness is particularly pronounced in the pelagic realm rather than in the benthos (Hillebrand, 2004). However, biodiversity patterns may be considerably more complex than was envisaged originally and deviations from this simple latitudinal trend can be found in various regions. As examples, diversity trends are not the same north and south of the equator (Gray, 2001, 2002), and regional ‘hotspots’ or coldspots’ of diversity exist for different marine organisms (Price, 2002; Worm et al. 2003). 5 CHAPTER I According to the Biodiversity Convention (www.biodiv.org), biodiversity is defined as ‘the variability among living organisms from all sources including inter alia, terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are a part; this includes diversity within species, between species and of ecosystems’. Definitions of biodiversity according to ecosystem structuring have been reviewed by Huston (1994), who emphasised the strong relationship between sampling scale and the processes that influence diversity. At small scales all species are assumed to interact with each other and compete for similar resources. Diversity within habitats has been called alpha diversity (Whittaker, 1960). When at slightly larger scales the sampling covers more than one habitat or community, it has been called between habitat or beta diversity (Whittaker, 1960, 1975). In contrast to this, gamma diversity describes patterns at a regional scale, when not ecological but rather evolutionary processes operate (Whittaker, 1960). According to Levin (1992) there is no single scale at which ecosystems should be described and it is difficult to scale up from the results of small-scale surveys to conclusions that are relevant to ecological patterns and processes at larger spatial scales (Thrush and Warwick, 1997). For the pelagic realm, Haury et al. (1978) were the first who emphasised the importance of scale by expressing the variability of plankton biomass by using a Stommel diagram, a three-dimensional presentation of biomass versus space and time. Stommel (1963) showed that there is an approximate linear relationship between the size and the longevity of eddy structures in the oceans. Any biological distribution patterns with similar space and time scales are, therefore, presumably determined by those hydrodynamic features. Deviating patterns were interpreted as being generated by biological processes like behaviour or recruitment or non-eddy related hydrographic features, such as frontal structures. Physical gradients in the open ocean: Implications on spatial variability Hydrographic features are the main abiotic factors influencing the structure of pelagic communities. Changes in distribution patterns and species diversity are expected to be sharpest where there are the strongest discontinuities in the physical environment. These regions characterised by ‘larger-than-average horizontal gradients’ are called fronts (Joyce, 1983). Fronts are generally supposed to act as barriers to distribution, although there are a 6 CHAPTER I variety of mechanisms of how parcels of water, containing biota, pass from one side to the other (Le Fèvre, 1986). Furthermore, horizontal gradients may be too weak to strictly limit distribution of most species, which make the definition of boundaries difficult. Therefore, fronts can be better described as leaky boundaries, rather than impermeable fences (Longhurst, 1998). Fronts are particularly pronounced where tidally mixed shelf water meets stratified water crossing the continental slope, and are often characterised by high biological productivity (Joyce, 1983; Le Fèvre, 1986). Tidal as well as shelf edge fronts are, however, not part of the present study that rather investigates, among other factors, the implications of oceanic fronts on the structure of species communities in the subtropical North Atlantic. In the open ocean two kinds of frontal systems are distinguished: (i) linear convergent zones, usually found at the confluences of two major currents, and (ii) linear features or filaments around or shed from meso-scale eddies. Especially at convergent zones enhanced levels of biomass have been observed (e.g. Le Fèvre, 1986). Some oceanic fronts like the conjunction between the cold Oyashio and the warm Kuroshio in the northern Pacific are particularly marked and represent a distinct boundary to the distribution of pelagic species (Hidaka et al., 2003; Nishikawa et al., 1995; Sassa et al., 2002a, b). A homologue feature can be observed in the Atlantic Ocean where the Gulf Stream faces the cold Labrador Current. The subtropical convergence zone of the North Atlantic is characterised by much less pronounced horizontal temperature gradients (Colton et al., 1975; Halliwell et al., 1991b; Voorhis, 1969). The convergence or ‘piling’ of water in these regions, as well as in the Pacific, is caused by a combination of surrounding atmospheric pressure systems, which produce winds that force Ekman transport. The subtropical convergence zone is produced by and thus situated between the eastward- flowing westerlies in the mid latitudes and the westward-flowing trade winds in the tropical latitudes of the North Atlantic. This combination of forces results in an eastward geostrophic flow and in a significant downwelling (Hanson et al., 1991). The concentration of floating organic matter along the front indicates an accumulation of surface-dwelling organisms (Pingree et al., 1974). Whether in these areas of enhanced frontogenesis the species composition and abundance varies or the front rather acts as a boundary to distribution remains largely unknown. The results of former studies were ambiguous. In the northern part of the Sargasso Sea, the thermal stratification of the euphotic zone is restricted to the summer period, which leads to north-south differences in net primary 7 CHAPTER I production (Ryther and Menzel, 1960). Correspondingly, higher abundances of mesopelagic fishes, fish larvae, or epipelagic copepods were found north of the subtropical convergent zone, but species compositions did not change consistently crossing the front (Backus et al. 1969; Boettger, 1982; Colton et al., 1975; John, 1984; Miller, 1995). Strong horizontal gradients related to meso- and large-scale eddies form the second frontal system found in the open ocean. Eddies originate e.g. from meandering streams, and large- sized and long-persistent cold-core or warm-core rings can be frequently observed along the major western boundary currents, the Gulf Stream and the Kuroshio. They significantly contribute to primary production in the subtropics and mid-latitudes