Nitrate to silicate ratio variability and the composition of micro-phytoplankton blooms in the inner-fjord of Seno Ballena (Strait of Magellan, 54°S)

The along-fjord variability of nitrate and dissolved silicate was studied in a silled fjord, Seno Ballena, in the Strait of Magellan during flood and ebb tidal phases in December 2007. The spatial and temporal variability of both nitrate and dissolved silicate were consistent with the dynamics of a tidal intrusion front previously described for this fjord by Valle-Levinson et al. (2006). During flood, maximum nitrate values were found seaward and close to the sill due to the upwelling of dense, nutrient-rich water by means of Bernoulli aspiration. Conversely, a sharp drop in surface nitrate landward of the sill was consistent with the sinking of saltier, nitrate-rich, dissolved silicate-poor upwelled waters under relatively less dense, nitrate-poor, dissolved silicate-rich surface waters after flowing landward over the shallow sill. The waters flooding over the sill were particularly enriched in nitrate but poor in dissolved silicate, (NO3−:Si(OH)4 ratio ∼5). The ratio tended to decrease landward of the sill, particularly during the ebb tide (NO3−:Si(OH)4 ratio <2), but the absolute Si(OH)4 concentration values were still very low, normally ranging between 2 and 3 μM Si(OH)4. Consistent with the low availability of dissolved silicate (DSi), only dinoflagellates (in the micro-phytoplankton size range) bloomed in nitrate-enriched (10 μM NO3−) waters at the base of a shallow halocline. Thus, it is likely that the variability of surface water nutrients and DSi in Seno Ballena results mainly from the interplay among tidally forced fertilization (the aspiration of subpycnocline water with a high NO3−:Si(OH)4 ratio), the input of continental waters (low NO3−:Si(OH)4 ratio), and local productivity. The combination of these factors explains the micro-phytoplankton composition observed landward of the sill on this and previous expeditions to the study area.