Zooplankton Aggregations at Coastal Fronts
An Introduction
A. Kimo Morris

The pelagic realm of the world’s oceans is dominated by a plethora of mostly small organisms collectively termed plankton. Plankton is dogmatically defined as organisms that drift or float with the prevailing water movement, and that lack self-motility. First used by Hensen in 1887 to describe these passively floating organisms, the word plankton is derived from the Greek , meaning “to drift” (Bougis, 1976). Virtually all marine animal taxa are represented in the plankton by some life history stage and, especially for meroplankton (planktonic only as larvae), the planktonic mode of life can be critically important. Yet despite the wide variety of phylogenetically distinct groups, there are certain general characteristics shared by plankton as a result of the paucity of microhabitats in the open ocean. Because of the absence of refuges, convergent evolution is a hallmark of plankton. Most plankton have evolved to be transparent and/or small. Living in a three-dimensional medium, unrelated taxa express long spines and appendages that act to increase drag, thus reducing sinking (e.g. copepods and larval decapods). Some plankton, such as siphonophores, utilize oil- and gas-filled floats to maintain near-neutral buoyancy. Still others, such as certain thaliaceans, maintain neutral buoyancy by excluding sulphate ions from their tissues (Bone, 1998). While the role that certain morphological features play in the life history of certain planktonic organisms is well established, fully understanding the ecology of these organisms and their role in connecting the pelagic realm with benthic and demersal organisms is still in its infancy.

A central goal of ecology is establishing relationships between variability in the physical environment with observed biological patterns (Krebs, 2002). This includes understanding the physical factors that influence patterns of growth, survival, reproduction, larval dispersal, and recruitment, and how physical and physiological adaptations and behavior act to maximize the observed patterns. In the marine environment, there are unique difficulties in measuring such patterns, particularly in pelagic coastal and open ocean habitats.

Oceanographic features, which might influence or control the abundance and concentration of plankton patches in the oceans, have been historically documented throughout the past century (e.g. Knauss 1957; and see Beebe 1926 for an excellent account). Zooplankton aggregations are known to result from water flow near headlands (Alldredge and Hamner, 1980), coral reefs (Carleton et al., 2001; Wolanski et al., 1989), and islands (Hamner and Hauri, 1981). While these examples of aggregation appear to be entirely mechanical, by the early 1980s, some researchers had strongly suspected that at least some aggregations may be behavioral in nature (Omori and Hamner, 1982). For example, it has been suggested that aggregations of copepods at the margin of river plumes respond to horizontal flow velocities and can occur in great abundance up to four orders of magnitude larger than in adjacent waters (Mackas and Louttit, 1988).

Large concentrations of plankton are common at this and other types of marine boundaries. For example, striking aggregations of zooplankton often occur at oceanic fronts, areas of particularly strong physical horizontal gradients, and convergences of adjacent water masses (Archer et al., 1997; Yoder et al., 1994). Physical differences in temperature as well as biological differences of chlorophyll-a on either side of a front are visible from satellite images (Yoder et al., 1994; DiGiacomo et al., 2002). At the ocean’s surface, fronts are frequently seen as highly visible lines of debris, often with seabirds that forage along the slick (DiGiacomo et al., 2002). Similarly, there is a disproportionate abundance of fish at fronts and these are sustained by aggregations of zooplankton. Fronts are advantageous also to larval fish and larvae of nearshore benthic invertebrates because frontal aggregations often transport these larvae to favorable sites for settlement and/or metamorphosis (Nakata, 1989; Govoni, 1993; Moser and Smith, 1993; Wing et al., 1995a; Wing et al., 1995b; Shanks et al., 2000).

Although there is a logical relationship between zooplankton abundance and fish density, the physical mechanisms by which zooplankton are aggregated have not been well investigated (but see Coyle and Hunt, 2000; Gaard, 1999; Graham et al., 1992). There are clear relationships between physical phenomena and corresponding aggregations of zooplankton. For example, Shanks (1983, 1985, 1988) observed concentrations of crab megalopae associated with surface slicks generated by internal waves. Also, accumulations of ichthyoplankton along estuarine fronts appears to be strongly associated with changes in tides (Narcross and Shaw, 1984; Largier, 1993; Kingsford and Suthers 1996). Transporting of aggregated ichthyoplankton is also mediated by physical mechanisms (Govoni, 1993; Moser and Smith, 1993; Nakata, 1989).

Because fronts aggregate zooplankton and ichthyoplankton, they also have a profound effect on gelatinous predators, such as jellyfish and ctenophores, which compete with and influence food availability for fishes. However, we are often taught that gelatinous marine organisms lack the ability to swim strongly against currents, and that for this reason, these tentaculate predators are invariably highly concentrated along fronts where convergent currents will concentrate them. However, new evidence suggests that these neurologically simply organisms can exhibit complex behavior (e.g. Hamner et al., 1994). Along with the fact that many gelatinous macrozooplankton have swimming speeds greater than the velocities observed at many fronts (Mileikovsky, 1973), it is entirely possible that the intense aggregations of large jellyfish at fronts may be due, at least in part, to behavior. Larson (1992) observed reproductive swarms of thimble jellies and showed that they maintained their position in a convergence zone by upward swimming. Further, owing to their delicate structure, gelatinous predators are virtually impossible to sample accurately with nets (Hamner et al., 1975), and hence only with in situ observations (either on scuba or with remotely operated vehicles) can one acquire robust data on abundance, density, or behavior. Thus, since little is know about means by which gelatinous predators are found at fronts, and how they maintain their position there, we know even less about their impact on fish or zooplankton populations at fronts, though the work of Hirota (1974), Purcell (1985), Schneider and Behrends (1998), Purcell et al. (1994), Purcell et al. (2000) and Weisse and Gomoiu (2000) has yielded compelling results.

In recent decades, advances in satellite technology, computer integration of in situ measurements, and global positioning systems (GPS) has resulted in greater accuracy in measuring large-scale patterns of patchiness (>10km). For example, strong oceanic fronts can be viewed with satellites (Yoder et al., 1994), and satellite images can be correlated with in situ data (Digiacomo et al., 2002). Also, since many zooplankton patches occur at the surface, certain species can be directly observed by aerial over flight (Jillett and Zeldis, 1985).

The purpose of this thesis is to observe how physical oceanographic processes influence the distribution of zooplankton in nearshore zones, and in at least one study, determine the degree to which behavior contributes to the horizontal pattern of zooplankton observed in the vicinity of fronts. Such information is critical to many applied aspects of marine and environmental sciences. For example, understanding the association between gelatinous macrozooplankton abundance and the presence of ichthyoplankton larvae will aid in the effective management of fisheries. Also, Marine Protected Areas (MPAs) are being scrutinized for their potential role in maintaining the abundance and genetic diversity of marine species. Understanding how certain ichthyoplankton and larvae of benthic invertebrates are aggregated in oceanic features and to what extent behavior is responsible for that aggregation will help to evaluate the best way to space MPAs. Recently, dispersal distance and larval duration have been shown to be very important to the success of MPAs (Botsford et al., 2001). Clearly, species with differing larval durations and swimming abilities will benefit to varying degrees from physical features that aid in dispersal and recruitment (Shanks, 1983; Shanks, 1985; Wing et al., 1995a; Wing et al., 1995b; Shanks et al., 2000).

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