Mixing Of Different Water Masses Research Articles

Blooms of the coccolithophore Emiliania huxleyi with respect to hydrography in the Gulf of Maine

We present results of oceanographic surveys of visually turbid blooms of the coccolithophore Emiliania huxleyi in the Gulf of Maine during the summers of 1988, 1989 and 1990. In each year, hydrographic stations within the blooms could be distinguished from non-bloom stations on a temperature-salinity diagram. In 1988 and 1989 the blooms were confined to the surface waters of the central western Gulf of Maine; T-S analyses showed they occurred in higher salinity surface waters at stations characterized by a well-defined upper mixed layer overriding a sharp pycnocline. Nutrients (not measured in 1988) were near depletion in the surface waters of both bloom and non-bloom stations in 1989, with surface phosphate being lower in the bloom waters (0.02–0.16 μM in the top 15 m) than in non-bloom waters (0.21–0.49 μM). Phosphate was not as low in the surface waters of the 1990 bloom. The bloom that year was much smaller in areal extent than in 1988 or 1989, and was limited to the northern part of the Great South Channel and western Georges Bank area of the Gulf of Maine. T-S analyses indicated significant mixing of different water masses in the area of the bloom in 1990, with the bloom being confined to those stations having less dense surface waters, of lower salinity, than the non-bloom stations. There also was evidence of a subsurface salinity minimum beneath the bloom waters in 1990. Blooms of E. huxleyi with surface expressions of visually turbid waters do not occur every year in the Gulf of Maine, and we discuss possible causative factors, specifically as related to the age or maturity of surface waters and macro- and micro-nutrient levels, that could facilitate bloom formation and which could vary between years.

Continuous shipboard and in situ measurements of pH and fluorescence in seawater

Discrete sampling can severely constrain investigations of physical, chemical and biological processes in the oceans. With the development of shipboard and in situ instrumentation for continuous measurements, it is now possible to identify important features, collect relevant samples, and modify the sampling protocol on the basis of an immediate feedback of data. Examples are presented here of instrument systems that provide continuous data in the horizontal and vertical domains. Two separate systems were used to record simultaneously chlorophyll α fluorescence and standard hydrological data. For coastal waters, we coupled a submersible data-logger to an in situ fluorometer. For deeper monitoring, a fluorometer and a pH sensor were deployed as part of a conductivity-temperature-depth (CTD) system. The advantages of simultaneous measurements are that data analysis is simplified, and changes in fluorescence (and pH) can be correlated with hydrographic features. Continuous surface fluorescence and pH measurements were made by diverting some of the water from a thermosalinograph intake to a Turner Fluorometer and a custom-built pH analyser. The temperature, salinity, pH and fluorescence were logged at approximately 1-s intervals and stored on a VAX computer. The simultaneous logging of these parameters enabled us to identify regions where the pH changed because of the mixing of different water masses. In the Australian Bight, the pH was directly correlated with fluorescence for 2 days over a distance of > 1000 km. At other times, the pH and fluorescence were not related, or occasionally were inversely related. The pH data were used to calculate the partial pressure of carbon dioxide ( pCO 2) in surface waters, and it was shown that changes in pCO 2 could be calculated with an accuracy of ≈ 0.7 ppm. The surface waters off northern Australia were estimated to be undersaturated with respect to CO 2 by ∼100 ppm.

Plankton

Today I must concentrate on just one of the many possible aspects of plankton research, one that 1 believe has a real future given the opportunity and the backing. This is: how the movement and mixing of water masses affect the plankton and, in turn, the effect this has on the other dependent communities and so on the fish themselves. We know that the mixing of different water masses produces conditions that are usually more productive than in either, and this can happen on a large scale, as in the convergences, or on a small scale in quite local areas. Sometimes the cause of the greater productivity is obvious—for example, off the coast of Peru where off-shore winds tend to drive away the depleted surface waters. These are replaced by the upwelling of nutrient-rich waters from below; some mixing takes place seeding the rich water with phytoplankton and resulting in one of the richest areas of production in the oceans and incidentally leading to an export of cheap fish meal that is having repercussions in the European markets.