Tesis doctoral de Arianna Bucci
Coastal microphytoplankton and microphytobenthos. Abstract abstract microalgae form diverse and complex communities in aquatic systems, which play a key role in biogeochemical cycles of different elements, like carbon and silicon, and are the base of marine food webs. The presence of a particular microalgae species in a system is associated to numerous physico-chemical parameters and to specific ranges of variability of those parameters. Moreover, its success depend on its ability to compete for resources, like nutrients. The importance of each environmental parameter and resource in driving the abundance and composition of microalgae communities may be different in different systems. Since microalgal growth depend on the in situ conditions, the environmental heterogeneity found in coastal systems is reflected by a concurrent heterogeneity in microalgae community patterns. To understand microalgae dynamics and their implications in nutrient biogeochemical cycles in coastal systems, different spatial-temporal scales of analysis are needed. the aim of the present thesis is to analyze the taxonomic composition and the spatial-temporal variability of microalgae communities (microphytoplankton and microphytobenthos) at different scales and in different coastal systems, and their relationships with environmental variables. In particular, we investigated the interactions of microalgae community with nutrient concentrations, and specifically with silicon biogeochemical cycle. the thesis comprises three complementary studies. In study 1, phytoplankton variability across a gradient of habitats over a tropical continental shelf, we investigated the species composition and abundance of the microphytoplankton over a shallow mesoamerican continental shelf (belize). Samples were collected during dry (december 2005) and rainy season (july 2005), along a presumptive environmental gradient extending from offshore blue water across a succession of continental-shelf habitats (i.E., Fore reef, lagoon patch reef, turtlegrass bed and island mangrove). Rich phytoplankton communities occurred in all habitats. Diatoms (127 species) were the most abundant organisms, followed by dinoflagellates (70 species), coccolithophorids (12) and silicoflagellates (1). There were marked seasonal differences in some environmental variables (seawater temperature, irradiance, salinity, rainfall and some nutrients). 1 coastal microphytoplankton and microphytobenthos. Abstract accordingly, we detected significant between-season differences in the taxonomic composition of phytoplankton, although variations in cell abundances were hardly detectable. Surprisingly, neither the analysis of nutrients in the water column (silicate, phosphate, nitrite+nitrate) nor phytoplankton abundance revealed a clear offshore to inshore gradient in the study area; the lowest values did not always occur in open waters and, even if they did, they increased in a non-lineal, complex pattern across shelf habitats. Each habitat showed a characteristic nutrient composition and a distinctive phytoplankton community, with between-habitat differences being most marked in july. The mangrove habitat, with the highest nutrient concentrations and noticeable abundance of dinoflagellates and coccolithophorids, was the most distinct. The studied environmental parameters did not fully explain the variability in phytoplankton composition, suggesting that additional factors are responsible for the marked between-habitat distinctiveness. in study 2, spatial-temporal patterns of nutrients and phytoplankton communities in nearshore habitats of the nw mediterranean, we examined one-year round patterns in variability of inorganic nutrients and microphytoplankton assemblages at the habitat and microhabitat scales in nearshore waters of a wave-exposed coast in the nw mediterranean. As habitats, we considered a) the nearshore, 37 m deep water column overlying a soft bottom and b) the demersal water layer associated to rocky bottoms. As microhabitats, we considered three depths (surface, mid depth, near bottom) within the water-column habitat and vertical versus horizontal rocky substrates within the hard-bottom habitat. Concentrations of silicate, phosphate and dissolved inorganic n compounds, as well as seawater n:si:p ratios, varied as a function of season and habitat, and to a lesser extent, of microhabitat. Phytoplankton abundance and species composition varied primarily associated with changes in nutrient (particularly silicate) availability. Diatoms were the dominant group through most of the year, with a major bloom in march and seasonal species replacement. However, in july, when abundance of total cells reached its annual minimum, dinoflagellates surpassed diatoms, probably as a consequence of silicon limitation, being zooplankton grazing postulated as a secondary control of diatom production. Silicon and phosphate were suspected to be limiting in this nearshore phytoplankton assemblage. When silicon in the system was separately analyzed as its dissolved (silicate) and particulate planktonic (frustules) forms, these forms showed reciprocal concentration patterns, indicating alternating processes of uptake by diatoms and subsequent frustule redissolution 2 coastal microphytoplankton and microphytobenthos. Abstract during most months, except from may through september, when total silicon showed minimum values. This minimum probably indicated that si was exported from the planktonic system to the bottom through either deposition, grazing or uptake by microphytobenthos and siliceous sponges. The boundary layer associated to rocky bottoms was characterized by a particular demersal (not benthic) phytoplankton assemblage, clearly distinguishable from that typically occurring in the water column over adjacent soft bottoms. A delicate nutrient-phytoplankton coupling, which varies at both habitat and microhabitat scales, seems to occur in these nearshore, coastal systems. in study 3, the role of benthic diatoms associated to maerl beds in the silicon cycle of the bay of brest (ne atlantic), we analyzed the taxonomic composition, abundance, biogenic silica (bsi) stock and silicon uptake rate of benthic diatoms associated to different shallow maerl banks in the bay of brest. The studied maerl habitat hosts diverse and complex assemblages of microphytobenthos, whose spatial variability in terms of specific composition and abundance was detected at a scale of few kilometers. We did not observe this same variability for the overlying phytoplankton communities. Benthic bsi stock ranged between 52.21 and 239.48 mmol bsi m-2 in summer, depending on the maerl bank, corresponding to 7-34 times planktonic bsi of the overlying 3 m-deep water-column. In autumn, the benthic bsi stock and diatom abundance were almost 10 times lower than in summer. The production rate of bsi by benthic diatoms associated to maerl banks, estimated from a 24 hour incubation of a a natural assemblage with 32si as a tracer, was 156.9 (±57.2) ¿mol si m-2 day-1 in autumn, when seawater silicate concentration was 14.70 ¿mol l-1, whereas it was only 3.4 (±1.4) ¿mol si m-2 day-1 in summer, with a silicate concentration of 0.76 ¿mol l-1. We estimated a specific uptake rate of 0.039 (±0.01) day-1 and 2.27í–10-5 (±8.36í–10-6) day-1, for autumn and summer, respectively. The high bsi stock and low productivity registered in the studied seasons highlight the need of additional studies to elucidate the possible limiting effect of a combination of nutrients. Our results remark the key role of benthic diatoms in the silicon cycle of this coastal system, which is comparable to that of phytoplankton, and highlights the importance of considering different habitats and spatial-temporal scales of variability when elaborating biogeochemical models of the silicon cycle in coastal systems. the study of microalgal communities in different coastal systems revealed the existence of complex relationships between environmental factors and local biological communities. The result is a great variability in microalgal patterns at different spatio-temporal 3 coastal microphytoplankton and microphytobenthos. Abstract 4 scales, which should be taken into account when developing studies and models in coastal systems.
Datos académicos de la tesis doctoral «Microfitoplancton y microfitobentos en sistemas litorales. diversidad, ecología e implicaciones en el ciclo biogeoquímico del silicio (coastal microphytoplankton and mycrophytobenthos. diversity, ecology and implications in silicon biogeochemical cycle)«
- Título de la tesis: Microfitoplancton y microfitobentos en sistemas litorales. diversidad, ecología e implicaciones en el ciclo biogeoquímico del silicio (coastal microphytoplankton and mycrophytobenthos. diversity, ecology and implications in silicon biogeochemical cycle)
- Autor: Arianna Bucci
- Universidad: Barcelona
- Fecha de lectura de la tesis: 26/11/2010
Dirección y tribunal
- Director de la tesis
- Manuel Maldonado Barahona
- Tribunal
- Presidente del tribunal: Javier Romero martinengo
- Marta Estrada miyares (vocal)
- (vocal)
- (vocal)