Journal of Plankton Research - current issue

Phytoplankton blooms in the Huon Estuary, Tasmania: top-down or bottom-up control?

Thompson, P. A., Bonham, P. I., Swadling, K. M. — 2008-07-22

The roles of "top-down" and "bottom-up" factors were investigated in terms of their influence on the diatom and dinoflagellate abundances in the microtidal, salt wedge Huon Estuary, Tasmania, Australia. Long-term (1996–2005) changes in Chl a, the peridinin:Chl a ratio and the abundance of autotrophic dinoflagellates were observed to coincide with the warming of regional surface waters. There were significant seasonal differences in pigment-specific net growth rates for Chl a, peridinin and fucoxanthin. Diatoms dominated the spring bloom when species such as Skeletonema costatum had the highest net growth rates and fucoxanthin-specific gross growth rates were ~0.9 day^–1. During late summer, the peridinin-specific grazing mortality was significantly less than the fucoxanthin-specific grazing mortality and dinoflagellates increased their dominance of the phytoplankton community. This late summer relaxation of grazing pressure on dinoflagellates was associated with a decline in the overall abundance of microheterotroph (MH) grazers and a peak in the abundance, biomass and estimated grazing rates of mesozooplankton. We suggest the composition of the autumn phytoplankton community was dependent upon a trophic cascade where mesozooplankton, such as Noctiluca scintillans, preyed upon MHs and reduced their grazing upon some species of dinoflagellates.

Heterogeneity of LSU rDNA sequences and morphology of Gymnodinium catenatum dinoflagellate strains in Bahia Concepcion, Gulf of California, Mexico

2008-07-22

We report the large subunit (LSU) rDNA sequences and morphology of several Gymnodinium catenatum dinoflagellate strains isolated from Bahía Concepción, Mexico. PCR amplifications of the D1–D2 fragment of the nuclear LSU rDNA gene resulted in a single product of ~889 bp. A phylogenetic tree was constructed from the partial LSU rDNA (350 bp) sequences from six strains of G. catenatum isolated from the bay and 12 isolates from around the world. Strains from the bay grouped within the G. catenatum clade; however, a constant characteristic of G. catenatum strains from the bay is that, at position 453, a single nucleotide polymorphism was observed, presenting cytosine instead of guanine. This single base polymorphism could indicate a mutation or genetic isolation from other G. catenum populations. It is suggested that the Western Pacific could be the ancestral population of this species. Significant differences were found between cell length (CL), cell width (CW) and CW/CL ratios in laboratory cultures in the late exponential growth phase. Average CL varied between 41 and 53.69 µm, with a range between 26.94 and 69.99 µm; average CW varied between 33.75 and 40.68 µm, with a range between 25.04 and 54.73 µm; the average CW/CL varied between 0.76 and 0.97 µm, with a range between 0.49 and 1.27 µm. Cingulum thickness was between 3 and 4.11 µm, with a range between 1.97 and 6.74 µm. Cell size varied with the length of the chain and position in the chain.

Light availability for Chlorobium phaeobacteroides development in Lake Kinneret

Rimmer, A., Ostrovsky, I., Yacobi, Y. Z. — 2008-07-22

The green sulfur bacterium Chlorobium phaeobacteroides forms a dense population in the oxygen-devoid layers of Lake Kinneret, Israel, from July throughout November. The typical vertical distribution of the species is characterized by a prominent peak at the center of the metalimnion, but the light available at that depth in Lake Kinneret is far below the values that enable maximal photosynthetic activity of C. phaeobacteroides. In this paper, we report on our findings regarding the characteristics of C. phaeobacteroides development in Lake Kinneret, both in space and time. We further quantified the available light in the metalinmion and suggest a model for the development of C. phaeobacteroides in the lake. Spatial analysis of C. phaeobacteroides suggest that a combination of diurnal north-west wind, diurnal oscillations of the metalimnion and the hours of sunlight result in a situation in which the metalimnion in some parts of the lake is exposed to slightly higher light intensities than the other parts of the lake, and therefore allows higher concentrations of C. phaeobacteroides to develop. The analysis of C. phaeobacteroides with time suggests that it can be described fairly well by the logistic growth model.

Successful establishment of the Ponto-Caspian alien cladoceran Evadne anonyx G.O. Sars 1897 in low-salinity environment in the Baltic Sea

Pollupuu, M., Simm, M., Pollumae, A., Ojaveer, H. — 2008-07-22

Our new results confirm that the Ponto-Caspian cladoceran Evadne anonyx was first found in the Gulf of Finland in 1999. In the Gulf of Riga, the first specimens were detected in 2000. The observed distribution pattern, together with the recorded increase in the population density of the species of ca. 10 times during the years 2000–2006 indicate the recent successful establishment of E. anonyx in the low-salinity conditions (<5 psu) of the NE Baltic Sea. Evadne anonyx and the native E. nordmanni are present in mesozooplankton community from May to October. In the seasonal cycle, the maximum abundance of E. anonyx may occur later in the season than that of E. nordmanni. As the fecundity of the alien E. anonyx significantly exceeds that of the native E. nordmanni, we suggest that population abundance of E. anonyx will very likely increase in future and the species may colonize new areas in the recently invaded ecosystem.

Hydromedusa blooms and upwelling events in the Bay of Panama, Tropical East Pacific

Miglietta, M. P., Rossi, M., Collin, R. — 2008-07-22

Seasonal upwelling events dominate the coastal areas of some regions of the Tropical East Pacific. The effects of upwelling on gelatinous zooplankton are largely unknown and undocumented for this or any region, although upwelling is known to shape phytoplankton dynamics. Small hydromedusae, the most widespread and diverse representatives of gelatinous plankton, are often neglected in plankton ecology as they are inconspicuous and escape direct observation. Their occurrence is seasonal and standard plankton sampling techniques easily overlook their blooms. In order to investigate whether upwelling affects hydromedusae dynamics, we monitored their abundance and diversity in the Bay of Panama, a region on the Pacific Coast of Panama characterized by seasonal upwelling events. Our results show that, although the number of species is relatively constant throughout the year, hydromedusa abundance in the Bay of Panama can be up to two orders of magnitude higher during upwelling than non-upwelling conditions. The difference in the numbers of hydromedusae between the two seasons is mostly due to temporally short medusa blooms that occur only during the upwelling season. Our results point to a link between hydromedusa blooms and upwelling events, and the increased productivity associated with them. The results are consistent with a scenario in which upwelling events act on the benthic colonies thus inducing medusa production.

Diversity and distribution of the mesozooplankton in the tropical Southwestern Atlantic

2008-07-22

Mesozooplankton species distribution and abundance were studied in the tropical Southwestern Atlantic, in neritic and oceanic regions off the state of Rio Grande do Norte, northeastern Brazil (3° to 6° S and 32°30' to 35°30' W). Samples were collected with a 120 µm mesh net towed horizontally at 40 stations along five transects in June 1986. A total of 151 taxa were identified. Copepoda (93 species) numerically dominated (78%) the community. The highest zooplankton abundance was recorded at a coastal station (1098 ind m^–3), and the lowest at the station farthest offshore (82 ind m^–3). The low abundance of zooplankton corresponded to oligotrophic water masses, and differences in abundance were locally affected by topographic upwelling. Three groups were distinguished: (i) oceanic with very low total abundance (<15 ind m^–3), (ii) a mix of neritic and oceanic species and (iii) coastal neritic group. Species diversity was high (mean 3.14 ± 0.42 bits ind^–1) and increased with the ecological stability of the area. The local oceanographic conditions, with a strong connection between the oligotrophic oceanic water masses and the enriched coastal water, are of paramount importance to the local food chain and patterns of zooplankton diversity.

Diet of round sardinella, Sardinella aurita, larvae in relation to plankton availability in the NW Mediterranean

Morote, E., Olivar, M. P., Villate, F., Uriarte, I. — 2008-07-22

The present study analyses for the first time the diet of larvae of the clupeid Sardinella aurita in the NW Mediterranean. Gut contents of larvae from first feeding (3.9 mm) to early postflexion stages (14.7 mm) were analysed. The type and abundance of ingested prey were related to the plankton composition in the environment in order to determine prey-type selectivity. The feeding incidence observed was among the highest reported for clupeid larvae, with higher values during the day (68.6%) than at night (7.7%), indicating that Sardinella aurita larvae are diurnal predators. The mean number of ingested prey was 3.3 (±2.51). The width of the prey increased from <30 to 410 µm with the development of the larvae, but the niche breadth did not vary with the size of the larvae. The diet was based on different stages of copepods (nauplii and postnauplii) and cladocerans (mainly Evadne spp.), though the preference for each type of prey varied, with higher preference for copepod nauplii in larvae <8 mm and a higher preference for Evadne spp. in larvae ≥8 mm. The copepod based diet of S. aurita larvae is very similar to that reported for Engraulis encrasicolus larvae in the Mediterranean, indicating a possible competition for food between the first-feeding larvae in situations of low prey abundance.

Dual control of the levels of photoprotective compounds by ultraviolet radiation and temperature in the freshwater copepod Boeckella antiqua

Garcia, P. E., Perez, A. P., Dieguez, M. d. C., Ferraro, M. A., Zagarese, H. E. — 2008-07-22

Photoprotective compounds (PPCs), such as carotenoids and mycosporine-like amino acids (MAAs), confer photoprotection to aquatic organisms against harmful ultraviolet-B (UVB) radiation. The natural variability of these compounds in zooplankton has been related to temperature, radiation and diet, but the ultimate mechanisms regulating the observed patterns in the field are still unclear. In this study, we analysed the variability of carotenoids and MAAs in a population of the calanoid copepod Boeckella antiqua in a shallow pond located in Northern Patagonia (Argentina). During our field survey, carotenoids and MAAs in B. antiqua varied without a clear seasonal pattern. Nevertheless, both groups of PPCs reached their maxima during spring and minima during summer. Inverse relationships were found between carotenoid concentrations versus temperature and irradiance. For MAAs, the same relationships were not significant. Tolerance experiments showed that mortality of B. antiqua was significantly influenced both by temperature and UVB dose, being more vulnerable at high temperature. We further investigated the effect of radiation regime and temperature on the bioaccumulation of PPCs in controlled laboratory experiments. We found that the concentrations of PPCs could be experimentally modified by manipulating radiation exposure and temperature. In addition, by breaking down the bioaccumulation processes into uptake and elimination, we were able to show that (i) the uptake rate was stimulated by photosynthetically active radiation (PAR)+UVA exposure and (ii) both uptake and elimination rates increased with temperature. Thus, the net accumulation (i.e. the balance between uptake and elimination), which ultimately dictates the concentration observed in an animal, could be either positive or negative depending on the specific combination of radiation exposure and temperature. The dual regulation of PPCs by radiation exposure and temperature should be considered in future efforts to reconstruct or predict the photoprotective responses of aquatic organisms to the past or future climate scenarios.

Benthic-pelagic coupling in sediment-associated populations of Karenia brevis

Sinclair, G. A., Kamykowski, D. — 2008-07-22

Nutrient delivery to populations of Karenia brevis in oligotrophic water columns in the Gulf of Mexico remains uncertain. Aggregations of K. brevis near the sediment–water interface suggest that cells derive nutrients from the sediment. Video of cells near the sediment suggest that cells either access nutrients that flux out of the sediment or migrate into the sediment pores where higher nutrient concentrations exist. Experiments tested K. brevis' ability to migrate into the sediment using chambers divided by a 100 µm mesh overlain with a thin layer of sediment. Since the diel vertical migration of K. brevis typically displays a nocturnal descent, experiments tested migration response at night in response to sub-sediment nutrient sources. The experiments suggest that while the sediment affects the progress of descending cells, migration occurs through thin layers of sediment and increases in response to elevated nutrient concentrations below the sediment. Since all cells found below the sediment had significantly higher C/N ratios than those remaining above the sediment, migration appears related to a cell's internal biochemical state. The vertical migration behavior of K. brevis may help alleviate bottom-up controls and permit populations to persist as vegetative cells near the sediment–water interface.

Summer planktonic copepod communities of Australia's North West Cape (Indian Ocean) during the 1997-99 El Nino/La Nina

McKinnon, A. D., Duggan, S., Carleton, J. H., Bottger-Schnack, R. — 2008-07-22

The community composition of pelagic copepods near Australia’s North West Cape (21° 49'S, 114° 14'E) was studied during the austral summers of 1997/98 and 1998/99. Most sampling occurred at a shallow (20 m) shelf station and a deeper (90 m) shelf-break station, though on four occasions a set of eight stations were occupied on a 36 km cross-shelf transect. During the El Niño conditions prevalent during the austral summer of 1997/98, episodic upwelling occurred causing intermittent high primary production. During the El Niño conditions of 1997/98, there was a little difference between stations in the spring (October–November), but communities differentiated later in the sampling season (December–February) with a more characteristic inshore community developing at the shelf station. In the La Niña conditions of 1998/99, the community at the shelf break was invariant, but the shelf community was mainly offshore copepods as a result of seasonal downwelling during the spring that was later replaced by an inshore community of more widely distributed species. Over 120 species of copepods were identified, of which the most speciose families were the Corycaeidae (22 spp.), Oncaeidae (>20 spp.), Paracalanidae (15 spp.) and the Oithonidae (11 spp.). Cross-shelf transects confirmed the existence of a distinct inshore community of copepods, dominated by small species of Paracalanidae and Oithonidae, and which was at least twice as abundant as those at the shelf break. In both summers, there was an onshore–offshore gradient in community composition, with the inshore stations characterized by small paracalanid and oithonid species.