Downwelling and dominance of autochthonous dinoflagellates in the NW Iberian margin: The example of the Ría de Vigo

Dinoflagellate blooms in coastal upwelling systems are restricted to times and places with reduced exchange and mixing. The Rías Baixas of Galicia are four bays in the NW Iberian upwelling with these characteristics where harmful algal blooms (HABs) of dinoflagellates are common. These blooms are especially recurrent at the end of the upwelling season, when autumn downwelling amplifies accumulation and retention through the development of a convergence front in the interior of Rías. Because oceanic water enters the Rías during downwelling, it has been hypothesised that dinoflagellate blooms originate by the advection and subsequent accumulation of allochthonous populations. To examine this possibility, we studied the microplankton succession in relation to hydrographic variability in the Ría de Vigo (one of these four bays) along an annual cycle making use of a high sampling frequency. The results indicated that upwelling lasted from May to August, with downwelling prevailing in winter. Microplankton during upwelling, although dominated by diatoms, evidenced a progressive increase in the importance of dinoflagellates, which achieved maximum abundance at the end of the upwelling season. Thus, diatoms characterised the spring bloom, while diatoms and autochthonous dinoflagellates composed the autumn bloom. Diatoms dominated during the first moments of the autumn downwelling and dinoflagellates were more abundant later, after stronger downwelling removed diatoms from the water column. Since the dinoflagellates selected by downwelling belonged to the local community, it is concluded that advection of alien populations is not required to explain these autumn blooms.     

Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset

In the retina of dusk-active European cockchafers, Melolontha melolontha, the linear polarization of downwelling light (skylight or light from the tree canopy) is detected by photoreceptors in upward-pointing ommatidia with maximal sensitivity at 520 nm in the green portion of the spectrum. To date no attempt has been made to answer the question of why these beetles detect polarization in the green. Here we present an atmospheric optical and receptor-physiological explanation of why longer wavelengths are advantageous for the perception of polarization of downwelling light under canopies illuminated by the setting sun. Our explanation focuses on illumination situations during sunset in canopied optical environments, because cockchafers are active at sunset and fly predominantly under canopies during their swarming, feeding, and mating periods. Using three simple atmospheric optical models, we computed the degree of linear polarization, the linearly polarized intensity of downwelling light, the quantum catch, and quantum catch difference between polarization detectors with orthogonal microvilli under canopies illuminated by the setting sun as functions of wavelength and solar zenith angle. Based upon these computations, we show that the green sensitivity of polarization detectors in M. melolontha is tuned to the high polarized intensity of downwelling light in the green under canopies during sunset, an optimal compromise between simultaneous maximization of the quantum catch and the quantum catch difference. We also briefly discuss how green-sensitive polarization detectors can function efficiently enough during the pre-feeding and egg-laying flights of cockchafers, which always occur prior to sunset and under the sky.     

Microprofiles of photosynthesis and oxygen concentration in Microcystis sp. scums

Abstract Oxygen microelectrodes were used to monitor oxygen concentration and rates of gross photosynthetic activity in Microcystis sp. scums which were formed and incubated under laboratory conditions. The depth of the photic layer, rate of photosynthesis, oxygen concentration and the location of the transition to anoxia in the scum depended on irradiance levels and colony size. Gross photosynthetic activity never extended below 2.5 mm depth in the scum. At high irradiance levels oxygen concentration in the upper 1.5 mm of the scum decreased and the oxygen concentration peak shifted to greater depth. Oxygen concentrations in scums composed of small colonies (<55 μm) were higher than concentrations in large colonies scums (> 300 μm) but small colonies showed stronger indications of photoinhibition. In a natural scum small colonies are presumably shielded from inhibitory intensities by larger colonies which will dominate the upper layers. Accumulation of low-light adapted, smaller colonies in deeper layers likely yielded a second peak in photosynthetic activity. In order to systematically discuss scums and scum formation a distinction is made in three different scum types.