Distribution, Population Dynamics, and Production of the Suprabenthic Mysid Mesopodopsis Slabberi in the Mondego Estuary, Portugal

Abstract

The spatial distribution, seasonal abundance, life history pattern, and production of the mysid Mesopodopsis slabberi are described from a warm-temperate estuary on the western coast of Portugal. Mysids were sampled from June 1996 to July 1997, using zooplankton and suprabenthic nets. The higher densities and biomass of M. slabberi were found in upstream areas from the south arm of the estuary. These areas consist of shallow waters with low circulation rates and high residence times, richer in phytoplankton than the rest of the estuary. Mesoopodopsis slabberi was abundant in October, November, and December and again in May, June, and July. No mysids were found in January and February, when salinity decreased to values below 5� and water temperature was below 10°C, and also in August, when oxygen levels declined and the water temperature reached values above 24°C. Morphometric relationships in M. slabberi were determined from freshly caught specimens. Significant positive correlations between total length (TL) and cephalic length (CL) (TL = 2.5 CL + 0.012) and between dry weight (DW) and total length (LnDW = 3.0298 LnTL -6.0229) were found. All size classes, from 0.95-11.36 mm, were equally represented throughout the year. Moreover, juveniles, immature females, mature ovigerous females carrying eggs or embryos, and mature resting females were found throughout the sampling period. As a whole, this pattern suggests that reproduction is continuous rather than seasonal. This, together with migrations, made identification and tracking of population groups impossible from the analysis of sizefrequency diagrams. The secondary annual production of M. slabberi was therefore estimated by the Hynes average cohort method. The annual production was 16.02 mg· m�3year 1, and the P/B ratio was 2.57.

Histochemistry and histology in planktonic ecophysiological processes determination in a temperate estuary (Mondego River estuary, Portugal)

“…It was verified that the sizes measured were in accordance with known trends for mysidacea (Allen, 1984;Mauchline, 1968Mauchline, , 1980Wooldridge, 1986), and particularly, M. slabberiat the study site (Azeiteiro et al, 1999). Even though the sizes of oocytes could not be compared to those mentioned in other studies, in absolute terms, the size relations between the various stages of oocytical development were quite similar to those observed in other studies.…”

“…This fact denotes the sheer contrasting characteristics of the two arms. During high tide, the North arm is 5-10 m deep, while the South is only 2-4 m deep, being almost totally silted in the upstream areas (Azeiteiro et al, 1999). As a consequence, the circulation in the latter depends largely on the tides and, to a lesser extent, on the freshwater discharge from a tributary-the Pranto River, which is controlled by a sluice located 3 km from the confluence with the Mondego.…”

“…Amongst the various possibilities (as, for instance, the mid-South arm, a commonly used location for past and present studies in the estuary), the Pranto River was chosen for the establishment of the sampling station, because the extensive work of Azeiteiro et al (1999), had previously demonstrated that the largest mysid populations were recorded in the upstream regions of the estuary, especially at the station located at the Pranto River sluice.…”

“…For these reasons, the species presented itself as a natural choice for the authors. In addition, a crucial factor enforced this choice: despite exhibiting an apparent continuous reproduction cycle, in the Mondego estuary, it almost disappears during the months of January, February and August, it possesses, on the other hand, a peak of abundance in October (Azeiteiro et al, 1999); these patterns are thus perfectly suitable to test the hypothesis presented for this study. For the reasons given above, a sampling strategy was designed so as to collect organisms from the months of August and February representing a state of reproductive impairment ("repressed" fertility-low population densities) and from the month of October, representing a state of full expression of reproductive capacity ("unrepressed" fertility-high population densities).…”

“…The species, therefore, plays a key role in zooplankton communities, accounting for total available carbon consumptions of up to 70% (Webb and Wooldridge, 1987); it is a selective grazer with a pronounced impact on phytoplankton communities (Webb and Wooldridge, 1990) and it is the most abundant among 14 species of mysids at the studysite (Azeiteiro et al, 1999). For these reasons, the species presented itself as a natural choice for the authors.…”