Calcidiscus quadriperforatus and Calcidiscus leptoporus as oceanographic tracers in Lisbon Bay (Portugal)

A study of the Calcidiscus genus in Lisbon Bay has revealed the presence of Calcidiscus quadriperforatus and Calcidiscus leptoporus. Samples were collected continuously on a weekly basis for four years (July 2001–May 2005) and on a cruise carried out in summer 2005, covering Lisbon Bay. Calcidiscus quadriperforatus and C. leptoporus developed in the same range of temperature and salinity, 11.5–21.5 °C and 26.9–36.3, respectively. The two species seemed to respond independently but had a co-occurring seasonal pattern nearshore, developing in different concentrations throughout the seasons. The maxima of both species occurred in low turbulent and nutrient enriched waters, which was also favourable for the development of larger coccospheres of C. quadriperforatus. The concentrations of both species decreased from 2001 until 2005, due to the intensification and persistence of upwelling and rainy periods. ate autumn until winter, mixed populations were observed nearshore, largely dominated by Calcidiscus quadriperforatus, in colder and nutrient enriched waters, while during spring, when temperatures begin to increase, only Calcidiscus leptoporus was observed, in mature upwelled waters. Calcidiscus quadriperforatus was considered more opportunistic than C. leptoporus. The availability of nutrients seemed to have influenced the size increase of C. quadriperforatus coccospheres. Both species developed offshore during summer, when the shelf is occupied by colder turbulent upwelled waters. A short downwelling episode in summer 2005, associated with two counter-rotating mesoscale eddies, responsible for a strong north-eastward flow of warm oceanic waters into the bay, allowed the development of C. quadriperforatus nearshore. Contemporary satellite images revealed the presence and onshore displacement of these waters and C. quadriperforatus and C. leptoporus distribution highlighted these conditions. The development of each species nearshore also gave indications of local oceanographic changes during winter and spring.