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| 2. |
- Kiorboe, T., et al.
(author)
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Intensive aggregate formation with low vertical flux during an upwelling-induced diatom bloom
- 1998
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In: Limnology and Oceanography. - 0024-3590. ; 43:1, s. 104-116
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Journal article (peer-reviewed)abstract
- The surfaces of most pelagic diatoms are sticky at times and may therefore form rapidly settling aggregates by physical coagulation. Stickiness and aggregate formation may be particularly adaptive in upwelling systems by allowing the retention of diatom populations in the vicinity of the upwelling center. We therefore hypothesized that upwelling diatom blooms are terminated by aggregate formation and rapid sedimentation. We monitored the development of a maturing diatom (mainly Chaetoceros spp.) bloom in the Benguela upwelling current during 7 d in February. Chlorophyll concentrations remained consistently high during the observation period (similar to 500 mg Chi m(-2)) and phytoplankton grew at an average specific rate of 0.25 d(-1). The diatoms were extraordinarily sticky, with stickiness coefficients of up to 0.40, which is the highest ever recorded for field populations. Combined with estimates of turbulent shear in the ocean such stickiness coefficients predict very high specific coagulation rates (0.3 d(-1)). In situ video observation demonstrated the occurrence of abundant diatom aggregates with surface water concentrations between 1,000 and 3,000 ppm. Despite the very high concentration of aggregates, vertical fluxes of phytoplankton were very low, with fractional losses <0.005 d(-1), and the aggregates thus seemed to be near neutrally buoyant. Losses due to copepod grazing were also low (similar to 0.025 d(-1)). Most of the aggregates were colonized by the heterotrophic dinoflagellate Noctiluca scintillans that feed upon diatoms in the aggregates. The system appeared to be in near steady state; specific diatom growth rate, coagulation rate, and loss rate due to N, scintillans feeding were all of the same magnitude (0.25-0.3 d(-1)) and the latter two varied in concert. Our observations provide only partial support for the population retention hypothesis because aggregate buoyancy and N, scintillans grazing efficiently reduced the vertical flux of aggregates in this system.
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| 3. |
- Tiselius, Peter, 1958
(author)
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An in situ video camera for plankton studies: design and preliminary observations
- 1998
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In: Marine Ecology-Progress Series. - 0171-8630. ; 164, s. 293-299
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Journal article (peer-reviewed)abstract
- A design for an in situ video camera for plankton observations is presented. A light weight aluminium rig supports a video camera with an opposing stroboscope which produces dark field images of plankton >0.3 mm. In the present configuration the camera can be used down to 100 m and provides video images of 10 to 40x magnification when viewed on a 14 " (ca 35 cm) monitor. Video recordings from the field show moderate aggregations of small copepods to the subsurface fluorescence maximum and at times indications of horizontal patchiness. The large copepod Calanus finmarchicus was found to aggregate 5 m below the fluorescence maximum at night. In comparisons with a traditional WP-2 plankton net, the video camera provided similar length frequency distributions while abundance estimates from video recordings were 10 to 20% higher than net estimates. II is concluded that the presented video camera offers excellent observations of the pelagic environment at an affordable cost.
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| 5. |
- Tiselius, Peter, 1958, et al.
(author)
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Colonization of diatom aggregates by the dinoflagellate Noctiluca scintillans
- 1998
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In: Limnology and Oceanography. - 0024-3590. ; 43:1, s. 154-159
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Journal article (peer-reviewed)abstract
- Abundance and vertical distribution of the dinoflagellate Noctiluca scintillans were studied during a diatom bloom in the Benguela current. Video observations showed the occurrence of abundant Chaetoceros spp. aggregates colonized by N. scintillans. The diatom aggregates were formed by regular coagulation of diatom cells and not by mucus feeding behavior of N. scintillans. N. scintillans can be positively buoyant, and estimates of encounter rates between N. scintillans and diatom aggregates during ascent demonstrates that this mechanism is sufficient to account for the observed colonization. The attached N. scintillans were feeding on the diatoms in the aggregates, as revealed by food-vacuole content. In the particular environment studied, N, scintillans appeared not to depend on aggregate feeding since shipboard experiments showed that clearance rates (10-20 mu l h(-1)) on unaggregated cells were sufficient to account for the high growth rates measured (up to 0.7 d(-1)).
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| 6. |
- Tiselius, Peter, 1958, et al.
(author)
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Effects of copepod foraging behavior on predation risk: An experimental study of the predatory copepod Pareuchaeta norvegica feeding on Acartia clausi and A-tonsa (Copepoda)
- 1997
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In: Limnology and Oceanography. - 0024-3590. ; 42:1, s. 164-170
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Journal article (peer-reviewed)abstract
- The effect of foraging behavior on predation risk was studied by exposing the two small calanoid copepods Acartia clausi and Acartia tonsa to 0 or 1 ppm (similar to 1,500 cells ml(-1)) of the diatom Thalassiosira weissflogii and to presence of the predator Pareuchaeta norvegica. In filtered water, predation rate was the same on the two species. In algal suspension, predation rate on A. clausi was half that in Filtered water and half that on A. tonsa. Video observations revealed distinct differences in motility of Acartia depending on algal concentration. Both species performed frequent short feeding bouts in algal suspension; nonfeeding copepods in filtered water alternately sank or adjusted their vertical distribution by stronger jumps. Jump frequency nearly doubled for A. clausi in filtered water. but no significant difference was observed for A. tonsa. To explain the predation, assuming that P. norvegica is a rheotactic predator, we developed a model of potential hydrodynamic disturbance associated with each foraging behavior. Increased encounter rate with P. norvegica caused by frequent strong jumps by A. clausi in the absence of algae could explain >40% of the observed increase in predation rate. For A. tonsa, jump frequencies and predation rates were similar in both food treatments, which is in accordance with the model.
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| 8. |
- Tiselius, Peter, 1958
(author)
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Short term feeding responses to starvation in three species of small calanoid copepods
- 1998
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In: Marine Ecology-Progress Series. - 0171-8630. ; 168, s. 119-126
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Journal article (peer-reviewed)abstract
- Three small, neritic copepods (Acartia tonsa, Acartia clausi, and Centropages hamatus) were exposed to short term (1 to 14 h) periods without food and their clearance over 2 h was measured. Clearance rates in controls consisting of copepods continuously exposed to 3500 cells ml(-1) of the diatom Thalassiosira weissflogii were 1.15 ml mu g(-1) dry wt d(-1) for A. tonsa, 0.65 for A. clausi and 0.35 for C. hamatus. When moved from filtered sea water to a suspension of 3500 T. weissflogii ml(-1), A. tonsa showed elevated (compared to controls) clearance rates after having been deprived of food for 6 h (+77 %) and 14 h (+44%). A. clausi only responded after 14 h of starvation (+60%) whereas C. hamatus showed a moderate response after 6 h without food (+14%). In day-night comparisons with A. tonsa, elevated clearance rates were significantly higher only during the day. Frequent estimates of clearance rates (20 min intervals) showed that the stimulating effect of food deprivation only lasted similar to 1 h in the case of 1 h of starvation but lasted more than 3 h after 14 h without food. Small species like the ones investigated here have more restricted vertical migration and may not leave the food-rich surface layer to avoid predation, as is commonly found in larger copepods. Instead, they have to balance food intake with predator avoidance continuously. The hunger responses observed in the study may allow the copepods to intermittently search for food or avoid predators and still maintain the same overall ingestion rate as constantly feeding animals.
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| 9. |
- Titelman, Josefin, 1973, et al.
(author)
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Vertical distribution, grazing and egg production of calanoid copepods during winter-spring in Gullmarsfjorden
- 1998
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In: Hydrobiologia. - 0018-8158. ; 376, s. 343-351
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Journal article (peer-reviewed)abstract
- The vertical distribution of copepods in relation to their potential food was examined in Gullmarsfjorden, Sweden (58 degrees 15.6' N, 11 degrees 27.2' E). Plankton distributions were determined from bottle samples at 5 m intervals on four occasions; 30 January, 28 February, 7 March and 10 April 1996. Potential food was crudely divided into chlorophyll containing protists (generally phytoplankton) and ciliates. Weak correlations between copepods and phytoplankton were found in January and April, while there were no correlations during the diatom spring bloom (February March) when phytoplankton were superfluous throughout the water column. Copepods and ciliates were never correlated. Ingestion as determined from gut fluorescence and egg production analyses suggested a higher degree of herbivory during the spring bloom than before and after. There was potential for copepod predation control of ciliate biomass throughout the study. Due to low copepod abundance in January through March estimated grazing pressure was not sufficient to control phytoplankton biomass until after the diatom spring bloom. Egg production rates were constant for Centropages hamatus (similar to 20 eggs female(-1) day(-1)) on all dates but more variable (1-26 eggs female(-1) day(-1)) for the other species. Temora longicornis and Acartia sp. both revealed their lowest fecundity during the bloom.
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