Authors: Maricela Yip and Pierre Madl

Salzburg, in the month of October 1999

The two-week, well prepared excursion was split into two major blocks, in an alternating sequence on a 2 day basis; a theoretical approach with mostly living species was given on the first day in the classrooms of the Center of Marine Research. Whereas, the second day involved the practical investigation at sites surroundig Rovigno, in accordance of the subject covered on the previous day.

Almost at the bottom of the trophic pyramid is the Plankton . It may be defined as a carpet of floating or drifting organisms with limited powers of locomotion and are transported primarily by prevailing water movements. Members of this group range in size from microscopic plants to jellyfish measuring up to 2m across the bell and includes the eggs and larval stages of nekton and benthos. Subdivisions of plankton include virio-plankton (pico- and nano-particles), bacterio-plankton (bacteria), phyto-plankton (microplanktonic flora), and zoo-plankton (micro- to meso-planktonic fauna or any benthic organism with a biphasic lifecycle).

Spring usually triggers the first larger biomass production (phytoplankton) followed by the increased production of zooplankton (mostly copepods, crustaceans, meroplanktonic larvae of benthic animals, and hydro-medusas) that peak in early summer.

• Phytoplankton or free floating planktonic plants, are usually single celled, (unicellular, filamentous, or chain forming species) that inhabit the photic zones of surface waters in open oceanic and coastal environments. Although unicellular forms comprise the bulk of phytoplankton, some green and blue-green algae are filamentous or even colonial, like diatoms and the blue-green algae. In addition, a number of diatoms and dinoflagellates produce chains of loosely associated cells. Phytoplankton encompasses a rather wide diversity of algal groups. According to their size, these diminutive autotrophs can be further categorized into ultra-, nano-, micro-, and macro-plankton.

• The Zooplankton are either herbivores, grazing on the phytoplankton, or carnivores feeding on other members of the zooplankton. Many of the zooplankton have some ability to swim and can even dart rapidly over short distances in pursuit of prey or to escape predators. They may move vertically in the water column, but like phytoplankton, they are also transported by currents. As a result of convergence zones and boundaries between water types, zooplankton exists in patches of high population density between areas that are less heavily populated. Such high population patches attract predators, whereas the more sparse populations between the denser patches preserve the stock. Turbulences and eddies favor dispersal of individuals from the densely populated patches to the regions sparsely populated.
  Some species obtain nutrition by direct uptake of dissolved organic nutrients (DON) but most species gather food via filter feeding or raptorial feeding patterns. Filter feeders employ tiny hairs or mucous coated surfaces to capture mainly phytoplankton. Carnivores such as raptorial feeders, seize and eat individual cells by removing a few selected prey; whereas, the grazing pressure by herbivorous zooplankton regulates the standing crop of phytoplankton populations.
  Besides predation and filtering, also physical-chemical conditions in marine waters affect species composition, abundance, and distribution of zooplankton. Thus, such microfauna must be capable to adapt to varying stresses associated with biological and physical-chemical factors (temperature, salinity, mass movements of water, and dissolved oxygen levels).

Foraminiferans of the phylum sarcodina and radiolarians of the phylum ciliata, are single-celled microscopic members of the kingdom protozoa. Foraminiferans are encased in compartmented calcareous coverings, while the radiolarian coverings are ornately sculputred and covered with delicate spines. Pseudopodia, many with skeletal elements, radiate from the cell and catch diatoms and small protozoa. After death, their coverings accumulate on the sea floor contributing to the bottom sediments.