Water column

About: Water column is a research topic. Over the lifetime, 13706 publications have been published within this topic receiving 496626 citations.

Primary Production in the Plankton Community of a Tropical Lake

Abstract: The primary production of Lake Lanao, Philippines, was studied over a 15—mo period by in situ application of C—14 and oxygen—difference techniques. Supporting data include weather, water chemistry, light penetration, and standing crop of both autotrophs and heterotrophs. A statistical treatment of production estimates precedes the presentation of data. Extensive comparison of the oxygen and C—14 methods indicates that the C—14 method as applied in Lake Lanao measures net primary production. Data from time—course experiments show no evidence of diurnal rhythms in the efficiency of photosynthesis per unit area of lake surface. Heterogeneity studies based on transect data indicate that at low to moderate levels of production, the probability that production at an index station will differ from the average for the lake on a given date by more than 30% is less than .05, while the comparable probability for high levels of production is .35. There is no significant difference between stations in mean primary production for the study period. Vertical profiles of photosynthesis exhibit light inhibition on all but the most overcast days. The threshold for inhibition at the surface is near 133 kerg/cm2°s during calm weather and somewhat lower in windy weather. The mean threshold for inhibition 1 m or more below the surface is lower than at the surface (101 kerg/cm2°s). The lake is exceptionally transparent (mean extinction coefficient, 0.38) considering its high productivity and has a vertical dispersion of production that is similar to temperate oligotrophic lakes. The characteristic is explained in terms of the low amounts of dissolved and suspended matter in the euphotic zone, high production per unit of standing crop, and great amount of mixing in the upper water column. Net primary production average 1.7 gC/m2°day, and gross primary production is 2.6 gC/m2°day. Autotrophs account for 80% of respiration in the euphotic zone. Factors controlling seasonal variation are related to resource supply rather than to temperature or biomass removal. Between 12 and 30% of seasonal variation in production can be accounted for by variations in incident light. Light limitation also occurs due to thickening of the zone of mixing during the circulation period and during storms. Nutrient supply is the dominant controlling factor during stratification. Nutrient depletion is relieved at frequent intervals by changes in the depth of mixing associated with storms. High sustained production on a low nutrient base is explained by rapid transfer of nutrients from the zone of decomposition back to the euphotic zone. General conclusions are drawn concerning the relative importance of seasonal and aperiodic variation in regulating the resource supply of temperate and tropical plankton communities.

Comparative study of spatial distribution patterns of the early stages of anchovy and pilchard in the NW Mediterranean Sea

Abstract: Anchovy and pilchard are the most abundant pelagic species in the NW Mediterranean Sea. They spawn in different seasons, subject to different environmental conditions: anchovy in summer, when the water column is stratified; pilchard in winter, when the water column is vertically homogeneous. The spatial distribution patterns of eggs and larvae of these 2 species are compared in relation to the main productive features in the region during their respective spawning seasons. The study was performed on the continental shelf off the Ebro River (NW Mediterranean) during June (for anchovy) and February (for pilchard). Sampling comprised a horizontal survey, designed to locate patches of eggs or larvae, followed by a Lagrangian experiment using stratified hauls to study the vertical distributions at different times of day in the water parcel tracked. While pelagic eggs and larvae of both species were present in the upper 70 m of the water column, we recorded differences in the preferential vertical distribution and migration patterns. Maximum concentrations of anchovy eggs and larvae were located in the upper 20 m; pilchard concentrations extended down to 10-40 m. Vertical displacements by the larger larvae at night occurred in both species, but with opposite patterns: anchovy larvae tended to aggregate in the upper 10 m, pilchard larvae exhibited greater dispersal at night, with a preference for levels below 30 m.