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Notes: Abstract Many pure samples of natural fecal pellets have been collected from mixed small copepods and from the pontellid copepod Anomalocera patersoni in the Ligurian Sea, using a specially designed pellet collection device. Sinking rates of fresh pellets and pellets aged up to 33 days have been determined at 14°C, the mean temperature of the essentially isothermal water column in the Ligurian Sea. Sinking rates of pellets collected during calm sea states increased with increasing pellet volume, but sinking rates of pellets collected during rough sea (Beaufort scale ≃6) showed little correlation with pellet size. Much of the variability in the sinking rate-pellet size relationships was the result of different pellet composition and compaction, but not pellet age. Pellets produced from laboratory diets of phytoplankton and phytoplankton-sediment mixes showed the expected wide variability in sinking rates, with sediment-ballasted pellets sinking much faster than pellets produced from pure algal diets; thus determination of vertical material fluxes in the sea using laboratory-derived fecal pellet sinking rates is unwarranted. Natural pellet sinking data for small copepods and A. patersoni have been combined with similar data for euphausiids, to yield sinking rates of roughly two orders of magnitude over three orders of magnitude in pellet volume. Pellets from small copepods sank at speeds too slow to be of much consequence to rapid material flux to the deep sea, but they undoubtedly help determine upper water distribution of materials. Recalculation of fecal pellet mass flux estimates from the literature, using our sinking rate data for natural small copepod pellets, yielded estimates about half those of previously published values. Earlier studies had concluded that small fecal pellets were of lesser significance to total material flux than fecal matter; our recalculation strengthens that conclusion. Pellets from large copepods and euphausiids, however, have the capability to transport materials to great depths, and probably do not substantially recycle materials near the surface. The fact that the majority of pellets which had previously been collected in deep traps by other workers were of a size comparable to pellets from our large copepods supports the contention that these larger pellets are the main ones involved in vertical flux.

Notes: Abstract The elimination of 3 radionuclides from Euphausia pacifica was measured over a 5 month period. The biological half-lives for 65Zn, 137Cs, and 144Ce, calculated after the euphausiids had ingested radioactive Artemia nauplii, were found to be 140 days, 6 days, and 7.5 h, respectively. The percentages of body burdens lost in molts were greatest for the fission products, 144Ce (21%) and 137Cs (7%), and least for 65Zn (1%). Elimination of the isotopes in the feces could not be followed because of the difficulty in collecting fecal material for analysis; however, 1 sample collected 2 months after the beginning of the elimination experiment had no measurable radioactivity. Loss of 65Zn from molts and time to disintegration of the molts were found to be temperature dependent over a 5° to 15°C range, and the sinking rate of molts was both temperature and salinity dependent. Calculations showed that, in areas in the North Pacific outside the influence of upwelling, percentage 65Zn loss from sinking molts (before disintegration of the molts) was likely to be the same throughout the year, since the molts would be exposed to about the same mean temperature in the water column in all seasons. Even though temperature structure in the upper layers changes with season, mean temperatures change very little when calculated over the sinking distance of intact molts. Intact molts would sink to slightly over 400 m in the absence of turbulence, and would lose 87% of their 65Zn by the time they reached this depth. Sinking molts thus might contribute substantially to the vertical transport of 65Zn in the sea. If loss of 65Zn in fecal pellets is assumed to be small under our experimental conditions, and molting loss is only 1% of 65Zn body burden, the major mechanism of 65Zn loss from euphausiids feeding on non-radioactive food must be isotopic exchange with the water. Approximately 96% of the initial body burden was eliminated over a period of 5 months.

Notes: Abstract The effects of temperature and body size on the intermolt periods (molting frequencies) of the North Pacific euphausíid Euphausia pacifica and the Mediterranean forms of Meganyctiphanes norvegica, Euphausia krohnii, Nematoscelis megalops, and Nyctiphanes couchii were studied under controlled conditions in the laboratory. Mean intermolt periods for E. pacifica and M. norvegica were inversely and linearly related to temperature, over temperature ranges which the euphausiids normally encounter in the sea. At higher temperatures there was a tendency for three size groups of M. norvegica to approach a minimum intermolt period independent of temperature. M. norvegica cycled for different time periods between 13° and 18°C molted regularly at mean frequencies which would be expected if the animals had been held constantly at the timeweighted means of the two experimental temperatures. The increase in mean intermolt period per unit weight was faster in small, fast-growing M. norvegica than in large, slow-growing adults. This relationship was corroborated by following the changes in the intermolt period of an actively growing individual N. couchii over an 11 month period. Neither feeding nor the time of year of collection affected the molting frequency as long as temperature and animal weight were held constant. No tendency was found for euphausiids of the same species and/or size, and from the same collection, to molt on the same night. Molting occurred at night 80 to 90% of the time for all species, over the temperature ranges normally experienced by the euphausiids in the sea, and over all animal weights tested. There appeared to be a weakening of the night-time molting rhythm at low temperatures. Although neither temperature nor anímal weight substantially affected the night-time molting rhythm, both affected the mean intermolt period. Therefore, both temperature and body size apparently act together to adjust the length of the intermolt period of each individual in increments of whole days, but they exert little control over time of molting within any 24h period. No information was obtained regarding the factors which specify night-time molting over daytime molting within any 24 h period; however, regulation of certain hormone activities is probably involved.

Notes: Abstract Influences of nutritional status on the photoinhibitory effects of ultraviolet-B radiation (UVBR: 290 to 320 nm) on the specific growth rates (μobs) and biomass of Phaeodactylum tricornutum were determined using nutrient-replete batch cultures and nutrient-limited continuous cultures. P. tricornutum cultures were exposed to UVBR doses representative of current mid-latitude and ozone-depletion intensities. Specific growth rates and biomass were inhibited from 2 to 16% by UVBR during nutrient-replete growth. However, no effect of UVBR on μobs or biomass was detectable when nutrient limitation exceeded the potential for limitation by UVBR. Thus, a competitive interaction appears to occur between macronutrient stress and UVBR stress, such that μobs and biomass will be determined by the most limiting factor. Our results suggest that measurable decreases in phytoplankton μobs and biomass from UVBR are most likely in nutrient-rich areas of the ocean, while these parameters may not be appropriate for measuring UVBR stress in regions of nutrient limitation.

Notes: Abstract After acclimation, the copepod Acartia clausi was allowed to graze for 5 days in a mixed suspension of two discrete size classes (species) of the chaining diatoms Thalassiosira spp. derived from continuous culture. Total particle numbers and particle size distributions of Thalassiosira spp. were stable throughout the 5 days, indicating that the effects of algal removal and modification due to grazing were balanced by algal growth. Grazer ingestion is the predominant process affecting all size classes of the smaller diatom population (T. nordenskioldii); however, both ingestion and chain modification are observed with the larger diatom (T. gravida). Although the greatest percentage removal of algal volume occurs in the largest algal size classes for each algal species, the greatest volume removal occurs at the modal peak (T. nordenskioldii) or just to the right of the modal peak (T. gravida). Flask-to-flask replicability of experiments was tested with the single-celled T. fluviatilis, and these tests were compared to the long-term experiments with T. nordenskioldii and T. gravida. Net particle removal occurs on both the large and small algal species in mixed suspension, not just on the larger-cell-side of the T. gravida distribution. Although 80% of the particles ingested are from the T. nordenskioldii distribution, 80% of the volume ingested is from the T. gravida distribution. The apparent multi-peak selection observed in our data is discussed in reference to two separate hypotheses and in light of other recent work pertaining to selective grazing by copepods.

Notes: Abstract Existing viewpoints and theories of selective grazing by copepods are briefly reviewed in order to formulate explicit hypotheses to be tested experimentally. Based on these hypotheses, a series of grazing experiments was run to determine (1) the extent of the selective ingestion capabilities of Acartia clausi and (2) how these capabilities were affected by previous feeding histories. Groups of copepods were separately preconditioned on a small diatom (Thalassiosira pseudonana), a large diatom (T. fluviatilis), or a plastic sphere. The ingestive behavior was then examined on various combinations of spheres and food particles. Spheres offered alone were not ingested. In mixtures of diatoms and spheres, the copepods avoided ingesting spheres intermediate in size between the sizes of the diatoms. The copepods either ingested particles on either side of the spheres, or ignored all particles less than the size of the largest spheres. The pattern observed depended upon the size of the preconditioning food. However, if the spheres were larger than the largest food particles, the copepods still selectively ingested the food particles. The above results demonstrate that A. clausi has a complex grazing behavior consisting of (1) more efficient grazing on larger particles within its particle-size ingestion range; (2) the ability to alter “effective” setal spacing to optimize feeding behavior (i.e., the ability to increase efficiency of capture of food particles, and to avoid non-food particles); and (3) the ability for post-capture rejection of non-food particles when they interfere with the ingestion of food particles on which the copepod has been preconditioned. The behavioral patterns observed depend heavily on the food preconditioning and the presence or absence of non-food particles. These results clearly indicate that a simple “mechanistic” explanation of selective grazing is insufficient.

Notes: Abstract Gut-evacuation rates were compared for feeding and non-feeding Calanus marshallae collected near Yaquina Bay, Oregon (44°37′N; 124°04′W) from June 1986 through May 1987. Evacuation rates were measured at four concentrations of Thalassiosira weissflogii from the decline of gut-pigment fluorescence following transfer of copepods to filtered seawater and from the loss of cells labelled with 68-germanium, a radioactive analog of silicon. There was no significant difference between gut-evacuation rates of feeding and non-feeding copepods over both short-term (20 min) and long-term (90 min) evacuation times. Furthermore there was no difference between rates obtained using either pigment or 68Ge as tracers of gut food-passage. These results are discussed in the light of possible “dilution” of tracer in the guts of feeding copepods due to mixing with unlabelled food. Gut-evacuation rates measured at food concentrations ranging from 500 to 4000 cells ml-1 were not significantly different, regardless of the technique employed.