Incubations

To understand how organic material and nutrients are channelled through the food web, it is important to quantify the rates (production, grazing, predation and sedimentation). Quan-titative information on feeding rates is particularly important since they represent the major transfers of biomass within ecosystems. The methods to quantify grazing (predation) of zooplankton are numerous and during the LOMROG II cruise, several methods (gut fluo-rescence, gut content analyses and pellet production) have been used. Most methods have strengths and weaknesses. The choice of which method to use depends on the type of zooplankton and the ingested food (herbivory, omnivory and carnivory). The combination of several methods will give us important information on different aspects of food and feeding.

Though some methods are more laborious than others (e.g. analyses of stomach contents) they are important since they can give information about food selection and prey-size prefe-rences (or limitations).

Animals for experiments (Table 6) were obtained from the upper 100 m using a WP-2 net with a 450 µm mesh size. For the in situ growth experiments a net with a 45 µm mesh size was used. The content of the cod-end was then transferred to a thermo box and brought to the main laboratory on the fore-deck. Following experiments were conducted:

Fecal pellet production, In situ pellet production, In situ growth rate experiments and Gut evacuation experiments. Water used in incubations was tapped from the seawater system (pumped from 10 m depth) located in the main lab onboard Oden. Animals for chemical analyses (carbon, nitrogen, isotopes, lipids and fatty acids) were collected from the same net hauls.

Figure 49. A refrigerator used to store incubations at in situ temperatures (-1 to -1.7°C). In the top Pareuchaeta sp., Calanus glacialis and C. hyperboreus are stored individually in 620 ml bottles.

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11.4.1 Pellets Production Experiments

Pellets (egested material) are the part of the food that has not been absorbed by the diges-tive system of the animal. For copepods, which have pellets covered by a membrane, col-lection of fecal pellets is possible. Moreover, the number of pellets must show a clear rela-tionship with feeding intensity and be independent of the type of food. To quantify ingestion, information about the pellet production rate and the relation between egested pellets and ingested food or absorption efficiency is needed.

Figure 50. Kajsa Tönnesson sorts copepods for experiments.

11.4.2 Pellet Production and Feeding Rates for Three Dominating Cope-pods

We conducted fecal production experiments for three of the most dominant copepods (Ca-lanus hyperboreus, C. glacialis and Metridia longa) on several stations during the cruise (25 stations with C. hyperboreus and C. glacialis, and 17 stations with M. longa). C. hyperbo-reus and C. glacialis females were individually transferred to 620 ml polycarbonate bottles containing 100 µm filtered seawater (Figure 48). M. longa females were transferred to 650 ml fecatrons (with false 400 µm mesh bottom). The bottles/fecatrons were then incubated for 24 h at in situ temperature (approximately -1 to -1.7^oC, Figure 49). After incubation the females length was measured and fecal pellets produced were counted and measured.

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11.4.3 In situ Fecal Pellet Production

The fecal pellet production of the copepod community was measured through short time incubations conducted at 5 stations (Table 6). On deck subsamples of the cod-end were immediately distributed into 5 fecatrons (PVC tubes with 400 µm false mesh bottom) filled with filtered sea water (Figures 51 and 52). The copepods were incubated for 1-2 hours, and then both copepods and fecal pellets produced were preserved in acidic Lugol’s solu-tion. At 5 stations, water samples for the quantification of suspended fecal pellets were collected at 3 depths (20, 40 and 100 m). The water samples were concentrated on a 20 μm sieve, fixed in 2 % acidic Lugol’s solution for post cruise analyses.

Figure 51. Five 1.5 l fecatrons used to measure the fecal pellet production of the whole cope-pod community.

11.4.4 Feeding Rates for Pareuchaeta sp.

Feeding rates for Pareuchaeta sp. were measured indirectly by estimating pellet production since experimental studies have shown a linear relationship between food intake and num-ber of pellets defecated. Within 1 hour of collection, individual Pareuchaeta sp. females or copepodites were transferred by pipette into 620 ml polycarbonate bottles filled with 64 μm filtered seawater. The bottles were incubated at 72 hours at in situ temperature (1 to -1.7°C). At the end of the incubations, the content was gently poured through a 50 μm sieve to collect animals and fecal pellets. The length of females or copepodites and the dimen-sions of the pellets were measured using a dissection microscope.

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11.4.5 Gut Evacuation Experiment

A gut evacuation experiment with Pareuchaeta sp. was performed at one station in the Amundsen basin (Table 6). Within 1 hour of collection, 10 females were gently transferred with a pipette to three fecatrons (10 females per fecatron), each standing in a beaker con-taining 64 μm filtered seawater. The beakers were then incubated in a refrigerator close to in situ surface temperature (-1 to -1.7°C) for 72 hours (until complete gut evacuation was reached). Every 30 to 120 min, the fecatrons were transferred to new beakers, and the beaker content gently poured through a 50 μm sieve to collect the pellets.

The gut evacuation rate will be determined by a regression between the number of pellets remaining in the guts of the 10 animals in each fecatron against time. To our know-ledge, this is the first report on fecal pellet production and gut evacuations rates for Pareu-chaeta sp. in this area. Predation impact can be calculated based on the feeding rate, Pa-reuchaeta sp. abundance, prey abundance and prey production. Complementary studies to show possible selectivity will be performed through gut content analyses after the cruise.

11.4.6 In situ Growth Experiments

In situ growth rate experiments with young stages of copepods (copepodites) were con-ducted at five stations during the cruise (Table 6). Within a few hours of collection a fraction (160 -200 µm) of the copepod community was transferred into cans filled with 64 µm fil-tered seawater. Four replicate cans were then incubated for six days at in situ temperature (-1 to -1.7^oC). At the end of the incubations, the content was gently poured through a 64 μm sieve to collect all animals. The samples were preserved in a 4% formalaldehyde/seawater solution for post cruise analysis.