Seasonal dynamics of ciliated protozoa and their potential food in an eutrophic estuary (Bay of Biscay)

Ciliate assemblages, together with phytoplankton and heterotrophic nanoflagellates (HNF) as their potential food, have been surveyed monthly along the salinity gradient of the Nervión River estuary from March 2000 to March 2002. A total of 135 taxa of ciliate have been identified corresponding to 20 orders. Scuticociliates, naked choreotrichs and oligotrichs were the most abundant groups, distributed throughout a broad range of salinity. In addition to these groups, which were the dominant taxa in the middle estuary, tintinnids were characteristic of the ciliate assemblages of the outer estuary whereas in the innermost part naked choreotrichs and oligotrichs decreased in number, being substituted by sessilids, hymenostomatids, peniculids and cyrtophorids. Total ciliate concentrations were among the highest reported in the literature for other estuaries and coastal waters, frequently reaching 10⁵ cells l⁻¹. Meanwhile, tintinnid concentrations were comparable with those of other estuarine and coastal waters. Based on a principal component analysis (PCA) three main groups were delineated, with ciliates and their potential food sources as variables. One was that constituted by freshwater ciliate such as sessilids, hymenostomatids, cyrtophorids and pleurostomatids, which appear in low numbers in the innermost part, mainly in winter. Another group contained diatoms, chlorophytes, flagellates, euplotids and scuticociliates, all of them broadly distributed along the estuary and reaching their maximum density in late spring–summer. The third group was made up of taxa mainly appearing at the seaward end such as dinoflagellates, cyanobacteria, haptorids, tintinnids, naked choreotrichs and oligotrichs, which reached the highest densities in summer and early autumn. Ciliates do not seem to be food limited in the Nervión River estuary or much controlled by metazoan grazing.     

Cyclic fluctuations of anoxia during Cretaceous time in the South Atlantic Ocean

The holes of the DSDP-IPOD program in the South Atlantic Ocean document two major anoxic events during Oxfordian to middle Albian times and secondly from late Cenomanian to Santonian times. The black shales formed during these two anoxic events differ in their rhythmicity and origin.During Lower Cretaceous time, the anoxic conditions resulted from the confined, euxinic nature of the basins. The rhythmicity of these black shales probably does not result from a global phenomenon (climatic or tectono-eustatic), but from local conditions resulting from the slender dimensions of the young ocean basin(s). The diversity and the diachroneity of the deposits from the south to the north precisely reflect the dynamics of the oceanic spreading.During Upper Cretaceous time, the anoxic conditions fluctuated in relation to a mid-water oxygen-minimum zone. The rhythmicity of black shale deposition seems to result from a global phenomenon, because of the widespread occurrence of the event. In the South Atlantic ocean, the cyclic fluctuations of anoxia were due to cyclic variations in the depth of the mid-water oxygen-minimum zone. There is no simple process to explain such rhythmicity. It probably results from the interplay of the three main variables which characterize the oceans at the time of the Cenomanian-Turonian boundary: the increased rate of sea floor spreading, high sea-levels and low water-circulation.     

Experiments on the resuspension of estuarine sediments containing benthic diatoms

A laboratory system was used to test the effect of water flow on the resuspension of mud and sand sediments and, specifically, benthic diatoms from the Ems estuary, The Netherlands. Current velocities generated by two rotating cylinders in a cylindrical tank were determined by a small float and a laser Doppler velocimeter. At low angular velocities, the amount of suspended matter increased linearly with angular velocity and the float current velocity. However, at higher angular velocities, the increase in current velocity was less because of the strong turbulence: concomitantly, the current velocity boundary layer (δ) became thinner and the suspended matter concentration increased rapidly. The dominant diatom species from the sandy sediment were suspended in two distinct groups, one of which consisted of the species Navicula aequorea, Navicula salinicola, Ophephora martyi and Opephora pacifica, and was more exclusively bound to sand grains than the other. The benthic diatom species inhabiting the silty sediment did not show this difference. The most important shortcoming in the experiments was the inability to determine the radial and vertical velocity components. This precluded reliable calculations of the shear stress. The data presented emphasize the importance of finding a method to determine the shear stress under experiments and field conditions so that direct comparisons can be made. Despite this it is assumed that, just as under the experimental conditions discussed, under natural conditions in shallow waters resuspension starts at current velocities as low as ca. 10 cm s⁻¹.     

von Liebig's law of the minimum and plankton ecology (1899–1991)

The Law of the Minimum was originally formulated by Justus von Liebig, as one of the 50 interlinked laws concerned with agriculture. The original writings of J. von Liebig often were misinterpreted by his successors. Brandt (1899) took this one law out of its context and proposed that limitation by nitrogen is a dominant factor in plankton ecology, far beyond its original application to agriculture. This was opposed by Nathansohn (1908) who suggested instead a dynamic balance of growth and loss terms. Towards validating, or eventually falsifying Brandt's hypothesis, Atkins, Harvey, Cooper and others developed the chemical methods necessary for re-defining ocean nutrient cycling and growth limitation. The major exception to these modern perspectives was the Antarctic Paradox of high nutrients and low chlorophyll which inspired Gran, Atkins, Harvey and Cooper to pioneer the concept of iron limitation. An exhaustive overview is given of efforts to define Fe in seawater and its controlling effect on in situ plankton growth, for the 1920–1984 period. Somewhat parallel work in the laboratory on single species of algae in chelation-controlled media has provided much insight, but is sketched only briefly. Martin and contemporaries developed the chemical methods necessary for defining the ocean chemistry of Fe and its role for in situ growth. These developments are sketched for the 1982–1991 period. Once again the Law of the Minimum and associated bold hypotheses served, albeit briefly, to bring a nutrient element in the forefront of research. This, and the recent awareness of CO₂ as rate limiting factor, underline the conclusion that advances in sciences often hinge on advances in technology, confirming Kuhn (1962). In this case the new analytical techniques developed by Atkins, Harvey, Cooper, Martin and their associates have proven revolutionary for plankton ecology. Some observations in plankton ecology may be reminiscent of the agricultural Law of the Minimum, but this would not warrant its direct application, beyond its original context and agriculture, to plankton ecology. Rather the net rate of increase of phytoplankton is the dynamic balance of multiple growth and loss terms, together also determining the biomass at given time and space.