Componentes de la Web semántica Los principales componentes de la Web Semántica son los metalenguajes y los

determine the quantity and quality of material ingested, rather than the bulk properties of the seston. In turn, because individual microplankton groups differ qualitatively, to understand the quality of the ingested diet requires specific

knowledge of the feeding behaviour of Calanus.

The process of feeding in Calanus was first described by Esterly (1916 c.f.

Marshall and Orr 1955a), and has subsequently received considerable attention (e.g. Harvey 1937, Gauld 1966). In brief, the feeding current created by the maxillipedes and maxillulary epipods carries particles forward into the filter-chamber where

particles are intercepted by the maxillary setae. Food items are then combed off the setae by the spines of the maxillulary endites and passed to the mouth. Numerous laboratory studies using cultured phytoplankton have demonstrated that copepods are capable of discriminating between different sized cells as a result of the structure of their filtering apparatus, and thereby preferentially ingest larger prey items (e.g. Marshall and Orr 1955a, Frost 1977, Berggreen et al. 1988). Early investigations into

the feeding mechanisms of Calanus proposed that the minimum distance between the

finest setules on the filtering appendages physically determined the minimum attainable prey size (Ussing 1938 cf. Marshall and Orr 1955b). It was suggested that

the smallest ingestible organisms for Calanus must be >5.7 µm, and this idea was

supported by the finding that cells < 10 µm were cleared by adults at much lower rates than larger cells (Marshall and Orr 1955b). However, using a mechanistic approach to feeding, Boyd (1976) suggested that in order to feed on small cells, copepods might simply increase the beating speed of the feeding appendages.

Cowles (1979) subsequently proposed that Calanus was capable of increasing the

fluid velocity across the particle capture appendages. According to the theory of particle motion in fluid flow (at low Reynolds number), this will increase the capture efficiency of smaller particles (Rubenstein and Koehl 1977). It has also been proposed that copepods can change the intersetule distances, thus altering their spectrum of retainable particles (Wilson 1973). Morphological evidence, based on electron microscope studies of the filtering apparatus of calanoid copepods (Friedman 1977 cf. Cowles 1979), supports this notion. Recently, Irigoien et al. (1998) conceded that at low food concentrations, small cells should be considered as

a possible food source for Calanus. Indeed, a range of zooplankters have been

reported to positively select cells <20µm (Perissinotto 1992). Meyer et al. (2002)

highlighted the importance of small cells in the diet of Calanus spp., and other recent

work has shown C. finmarchicus capable of efficiently grazing cells ~5µm (Huntley

1981, Hansen et al. 1994b, Nejstgaard et al. 1997), with such cells maintaining optimal reproductive output (Bamstedt et al. 1999) and constituting the majority of the total carbon ingested at times (Levinsen et al., 2000b).

When copepods are presented a natural microplankton assemblage, trends in feeding selection are not always apparent and sometimes contradictory. For example,

the diet of Calanus spp. in both the Labrador Sea and in the English Channel was

reported to closely reflect that of the available microplankton community (Huntley

1981, Irigoien et al. 2000a), whereas diatoms were strongly selected in the Norwegian Sea (Meyer-Harms et al. 1999). Despite such inconsistencies, a common finding from studies offering natural microplankton assemblages is that in general, epipelagic copepods clear microzooplankton at higher rates than autotrophic cells (Stoecker and Egloff 1987, Gifford and Dagg 1991, Atkinson 1994, 1995, 1996, Verity and Paffenhofer 1996, Irigoien et al. 1998, Zeldis et al. 2002, Bollens and Penry 2003) and strong positive selection is typically shown towards motile prey (e.g. Nejstgaard et al. 2001b, Bollens and Penry 2003).

According to recent prey switching theory (Saiz and Kiorboe 1995, Kiorboe et al. 1996; see also Greene 1988, Jonsson and Tiselius 1990, Tiselius and Jonsson 1990), when the environment is dominated by non-motile prey, copepods adopt a suspension feeding mode in which food items are entrained into the feeding current created by rhythmical beating of the maxillipedes (see Marshall and Orr 1955b). However, the ‘jump’ escape response typical of ciliates under attack by copepods has been shown to be effective in reducing their mortality (Broglio et al. 2001, Jakobsen

2001). Thus, when Calanus adopts a suspension feeding mode, ciliates may be

expected to be under-represented in the diet relative to the food environment unless;

a) the escape response is ineffective against Calanus’ feeding current or; b) upon

detection (mechanoreception) of ciliates (see Visser 2001), Calanus briefly switches

to a raptorial mode of feeding. Jakobsen (2001) showed that the level of water disturbance required to elicit an escape response in ciliates was lower than the disturbance created by the feeding current of small copepods. Therefore it is unlikely

that they would be ingested if Calanus was simply suspension feeding. Upon

detection of motile prey, Calanus has previously been observed to switch from the

characteristic suspension-feeding mode to one of active predation (Conover 1966). Subsequent quantitative experimentation has supported these early observations,

illustrating differential feeding behaviours for non- and motile prey in Calanus

(Landry 1980, Landry 1981).

Studies that only consider the ingestion of autotrophic material (e.g. using the gut fluorescence technique) have frequently shown that the amount of ingested carbon fails to fulfil the metabolic demand, and it is often suggested that heterotrophic microzooplankton are consumed to fulfil this shortfall (Dagg and Walser 1987, Gifford and Dagg 1991, White and Roman 1992, Atkinson 1996, Razouls et al. 1998, Mayzaud et al. 2002a, b). Indeed, copepods derive substantial

proportions of their daily rations from ciliates and other heterotrophic protists (Gifford and Dagg 1991, Kleppel et al. 1996, Rollwagen Bollens and Penry 2003). In addition to their quantitative importance, there is an increasing amount of information illustrating the qualitative importance of microzooplankton in the diet of copepods (Stoecker and Capuzzo 1990, Kleppel 1993). Corner et al. (1976)

demonstrated that the copepod C. helgolandicus had a significantly higher

assimilation efficiency for nitrogen when feeding carnivorously. The faster and more efficient utilisation of the digested components was attributed to the strong

similarities between the biochemical compositions of C. helgolandicus and their

metazoan prey (barnacle nauplii). Both ciliates and dinoflagellates are relatively rich in nitrogen when compared to diatoms (Stoecker and Capuzzo 1990) and it has been suggested that this renders microzooplankton of higher nutritional quality (Gifford and Dagg 1991). Whilst it is acknowledged that the biochemical composition of cultured algae varies depending on the conditions under which it was grown (Ackman et al. 1968, Chuecas and Riley 1969, Dunstan et al. 1993), for a given cell volume, cultured dinoflagellates are estimated to provide 2-6 times more protein, 2.5-3.5 times more carbohydrate, and 1.1-3 times more lipid that diatoms (Hitchcock 1982). Ciliates contain 1.8 times more carbon times that of a dinoflagellate of equivalent volume (Ohman and Runge 1994). Indeed, there appears to be a causal

relationship between in situ copepod egg production and the abundance of

microzooplankton (Runge 1985, White and Roman 1992, Ohman and Runge 1994, Jonasdottir et al. 1995, Pond et al. 1996). Additions of ciliates or rotifers to mono- specific algal diets of copepods causes a reduction in development time, increases the longevity of females, and also increases egg production (Stoecker and Egloff, 1987, Bonnet and Carlotti 2001). This is possibly because protozoa are an important source of essential nutrients, particularly specific PUFAs (Stoecker and Capuzzo 1990). Therefore, in addition to providing information about the physical and behavioural aspects of copepod feeding, determining patterns of food selection can also be used

to provide information about the potential quality of the diet of Calanus.

1.4.1. Calanus and detritus. Detritus features as a dietary component for some copepods (Heinle et al. 1977, Kosobokova et al. 2002, Schnetzer and Steinberg

2002 Kattner et al. 2003), and Calanus has been observed to ingest dead

phytoplankton cells and copepod fecal pellets (Paffenhofer and Strickland 1970,

Paffenhofer and Knowles 1979). However, the extent to which Calanus ingests “marine snow”, fragile organic aggregates that are formed by the coagulation of smaller particles such as phytoplankton and fecal pellets (Alldredge and Silver 1988), remains largely unknown. This is primarily because examining this question remains methodologically complex (Dilling and Brzezinski 2004). Early

experimental work demonstrated that Calanus was unable or unwilling to ingest

marine snow (Paffenhofer and Strickland 1970), but more recently, Dilling et al. (1998) suggested that marine snow was ingested in the absence of other food

sources. However, in these experiments Calanus adopted a ‘benthic feeding mode’,

only ingesting material that had collected on the base of the experimental containers (Dilling et al. 1998). Since this situation does not occur in the open ocean, marine

snow is not considered to play an important role in the ingestion of Calanus (Irigoien

et al. 1998, Meyer-Harms et al. 1999). It is likely that the mechanical process of

Calanus swimming and feeding in open water causes the fragile aggregates to

fragment, deeming them too small for efficient ingestion (see Dilling and Alldredge

2000). Calanus is considered to feed primarily on viable microplankton cells

(Kleppel 1993, Harris 1996), and therefore determining total POC levels will provide little information about the quantities of available food unless the majority of this POC is comprised of cells that are readily ingestible.

1.5. The use of characteristic ‘biomarker’ fatty acids. It is well established