bedford institute of oceanography 2001 in review - Région des ...

SCIENCE ACTIVITIES / Ocean SciencesOcean SciencesTales of the Unexpected from the Scotian Shelf- Erica Head and Leslie HarrisSince 1998, the Ocean Sciences Division has been collecting hydrographicdata and chemical and biological samples at selected sites onthe Scotian Shelf. The collections are made each spring and fall as partof the Atlantic Zone Monitoring Program (AZMP). This programwas set up to observe climate-related changes in environmental andbiological conditions of the lowest levels of the food chain (plankton)that might affect the status of higher trophic levels includingcommercial fish species. Zooplankton, the principal vectors for thetransfer of energy from the primary producers (phytoplankton) tothe higher trophic levels, are collected using plankton nets fitted witha 0.2mm mesh. This article describes several recent unusualzooplankton observations.Occurrence of a bloom of an exotic phytoplankton speciesOver the last few decades the introduction and geographic spread ofexotic, or non-indigenous, species has increased dramatically. Thechief mechanism is thought to be the ballast water exchanges carriedout by cargo ships. One well-known example is the introduction ofzebra mussels into the Great Lakes. Introductions of non-indigenousphytoplankton are often unnoticed until they reach “nuisance” status.Here,we are reporting the occurrence of a bloom of a species of nonindigenousdiatom on the Scotian Shelf, described as a nuisancespecies elsewhere, which has had no recorded effects here.In the spring 2000, plankton tows from most of the central andwestern Scotian Shelf sampling sites contained high levels of phytoplankton.This is not in itself unusual, since many phytoplanktonspecies, especially diatoms, form long chains of cells that can getcaught up in the 0.2mm mesh. When looking at our samples underthe microscope, however, we found that this was not the type ofphytoplankton usually seen (Fig. 1). Instead, it was in the form oflarge individual diatom cells, shaped like petrie dishes, which hadboth diameter and height of approximately 0.25mm.The diatoms were identified as Coscinodiscus wailseii (J. Martin,DFO, St. Andrews). This species was originally reported in only twoareas: the Pacific coast of North America and Japanese coastal waters.Since the late 1970s, however, C.wailseii has established itself as animmigrant in the waters around the United Kingdom and in the NorthSea. C. wailseii cells may be too big for many zooplankton, such as thecopepods in Fig. 1, to eat so that blooms may cause disruptions in thefood chain at its lowest level. Also, it has been reported that C. wailseiican clog fishing nets, by producing large amounts of mucilage, andthat it is noxious to Nori, a seaweed harvested in Japan. As yet,however no complaints have been reported of either of these potentialnuisance effects from around our coast.The history of occurrence of C. wailseii on the Scotian Shelf ispartially documented through Continuous Plankton Recorder (CPR)records. The CPR is a towed body, deployed from commercial ships,that collects, preserves, and saves plankton samples. C. wailseii did notFigure 1. Plankton net sample collected on Browns Bank in April 2000. Contents ofthe diatoms appear shrivelled due to chemical preservation. Preservation alsodestroys the colours of the phytoplankton and zooplankton (copepods)– photo by Dan Jackson.occur on the Scotian Shelf in the 1970s, but it had arrived by the early1990s. It is an interesting thought that the commercial vessels thatconduct long-term monitoring of exotic species invasions, such asthis one, may also contribute to their occurrence.Strange copepod mating behaviour on the Scotian ShelfOne species of copepod, Calanus finmarchicus, dominates thezooplankton in spring and early summer. Individuals spend thewinter as pre-adults in the deep waters surrounding the shelf or in theshelf basins, but as spring arrives they swim up to the surface to moultand develop into sexually mature males and females. Dr. CharlesMiller (Oregon State Univ.) has described the mating process. Newlymoultedfemales perform a “hop and sink” swimming routine, whilereleasing a trail of pheromones that attracts the males. The malesswim in broad horizontal loops until they get near a female at whichpoint their loops become smaller and three-dimensional. Someencounters do not result in successful mating, but in those that do,the male clasps the females and transfers a sac of sperm, called a spermatophore,to the genital segment of the female’s tail. This is thenormal routine, but samples collected from sites on the northeasternshelf (off Louisbourg) in April 2000, contained products of someapparently sexually deviant behaviour.Firstly, we saw juveniles of another related copepod species(Calanus hyperboreus) with spermatophores attached to one of theirtail segments (Fig. 2). These juveniles were approximately the samesize as female C. finmarchicus. Male C. finmarchicus were the only18 / BIO-2001 IN REVIEW