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The focus of the research is the trophic role of small (bell diameter < 5 mm) hydromedusae. Medusae are important, often highly selective, planktonic predators that can strongly affect standing stocks of metazoan zooplankton as well as fish eggs and larvae. The feeding rates, prey selection patterns and the underlying feeding mechanisms that have led to the success of large, conspicuous medusae have been well studied. However, most medusae are small. These small medusae are abundant, taxonomically diverse, and often have cosmopolitan distributions. Yet their feeding ecology is virtually unknown despite their status as the largest and most diverse group of gelatinous zooplankton in the sea.
The following hypotheses were tested:
(1) Small hydromedusae feed in ways that enable them to utilize protistan prey, including microzooplankton as well as autotrophic protists typically regarded as phytoplankton. Consequently, these hydromedusae are omnivores utilizing microplankton prey rather than strict predators on metazoans.
(2) The combination of high seasonal abundances of these small medusae in coastal waters and their omnivorous diet allows them to function as significant grazers of phytoplankton during periods of peak seasonal production.
Specific objectives of the research were:
(1) Quantification of the ecological impact of small hydromedusae on their prey community. This objective will be accomplished by measuring the temporal distribution and abundance of hydromedusae and their potential microplankton and metazoan prey, quantifying feeding rates and prey selection, and confirming that the prey ingested are utilized for growth by the medusae.
(2) Quantification of the mechanical basis of prey selection by the target organisms. This objective will be accomplished by evaluating the sequential components of feeding for each of the target species, predicting prey selection patterns from measured encounter, retention and capture efficiencies of different prey types, and comparing predictions derived from this analysis with empirical results from in situ feeding and prey selection studies.
Field Sampling: Field sampling in Narragansett Bay was done twice weekly in 2005 and once weekly in 2006 during the hydromedusan growth season. Hydrographic data was collected using a HydroLab Datasonde 4 profiling package outfitted with sensors for depth, conductivity,and temperature. The potential prey field of hydromedusae consists of mesozooplankton (>200 µm) and microplankton (autotrophs and heterotrophs <200 µm). Mesozooplankton samples include both the hydromedusae and their potential mesozooplankton prey. Mesozooplankton were collected by vertical hauls of a metered 64 µm-mesh 0.5m diameter ring net. Samples were preserved in 4% buffered formaldehyde. Samples for analysis of autotrophic and heterotrophic microplankton were collected using a Niskin bottle deployed at targeted depths, typically 3 depths perstation (surface, mid-depth and bottom of the well-mixed water column). Samples were preserved with 10% (vol/vol) acid Lugols solution, settled, and then processed by inverted microscopy (Gifford and Caron 2001).
Microzooplankton abundance is displayed as number/liter, the other groups are number/cubic meter.
Sensors for temperature, conductivity, salinity, specific conductance, TDS, pH, ORP, dissolved oxygen, turbidity, chlorophyll a, blue-green algae, Rhodamine WT, ammonium, nitrate, chloride, ambient light (PAR), and total dissolved gas.
This package was outfitted with sensors for depth, conductivity,and temperature.
A Niskin bottle (a next generation water sampler based on the Nansen bottle) is a cylindrical, non-metallic water collection device with stoppers at both ends. The bottles can be attached individually on a hydrowire or deployed in 12, 24 or 36 bottle Rosette systems mounted on a frame and combined with a CTD. Niskin bottles are used to collect discrete water samples for a range of measurements including pigments, nutrients, plankton, etc.
Deployed at targeted depths, typically 3 depths per station (surface, mid-depth and bottom of the well-mixed water column)
A Plankton Net is a generic term for a sampling net that is used to collect plankton. It is used only when detailed instrument documentation is not available.
64 µm-mesh 0.5m diameter ring net with flow meter.
depth of water
abundance of animals: microzooplankton abundance = #/L; all other groups = #/m^3
local day and decimal time, as 326.5 for the 326th day of the year, or November 22 at 1200 hours (noon)
year of sampling
month of sampling, local time
day of month
Latitude - North is positive
Longitude - West is negative.
Biological and Chemical Oceanography Data Management Office.
Funded by the U.S. National Science Foundation