Mesoscale, finescale and microscale distribution of micro- and nanoplankton in the Irish Sea, with emphasis on ciliates and their prey

We examined the degree of mesoscale (km), finescale (m), and microscale (cm ) patchiness of ciliates and their prey in waters of varying hydrographic c onditions. Samples were taken throughout the water column, along a transect across the Irish Sea (54 degrees N), at scales ranging from 0.15 to 10(5) m. We examined physical, chemical, and biological characteristics. The east ern and western Irish Sea were stratified, with a pycnocline at similar to 20 to 30 m. The central waters were mixed and had adjacent frontal regions. Euphotic depth was similar to 20 to 35 m. Generally, the upper waters were nitrogen-limited, with elevated levels associated with frontal regions and deeper waters. Microphytoplankton exhibited fine-mesoscale patchiness: dia tom numbers were low in stratified waters, with higher levels in mixed and frontal regions; dinoflagellates were abundant in subsurface waters near th e fronts. Nanoflagellate numbers and biomass decreased with depth below the pycnocline, and exhibited microscale distribution in upper waters; these m icropatches may provide increased food levels for ciliates. Microscale dist ribution of ciliates was rare and only occurred at mixed/frontal sites; fin escale ciliate patches were a more prominent feature of the water column. T hese finescale patches can be composed of a variety of taxa but can also be virtually monospecific. Finescale patches may produce localised regions of high productivity that is available to fishes and copepods, but may also b e a sink, as patches can be short-lived and thus unavailable to predators.