Response of a Mediterranean phytoplankton community to increased nutrient inputs:a mesocosm experiment

Abstract

The response of the biomass and primary production of a coastal NW Med~terranean phytoplankton community to a gradient of nutrient inputs was examined in a large-scale mesocosm nutrient enrichment expenment over a period of 20 d in summer. The mesocosm units (33 m3) received N, P and Si at a stoichiometric ratio of 20 N:? Si:l P, at the normal nutrient loading rate at the site (5 mm01 N m-' d-' and 0.25 mm01 P m-2 d-l), and at 0.5, 2, 4, 8 and 16 times the normal nutrient loading rate. The biomass and production of the phytoplanktonic community was also examined in a mesocosm unit to which no nutrients were added Substantial differences in phytoplankton biomass were observed 4 to 12 d after the initiat~on of the experiment, when nutrient inputs were increased more than 4-fold above the normal nutrient loading rate The b~omass of the community increased to a maximum of 40.8 pg chlorophyll a I-' (200-fold above the mean initial value) at the greatest nutrient inputs. The biomass increase was largely attributable to an increase in the microphytoplankton biomass (Chaetoceros sp. and Licmophora sp.), while picophytoplankton increased their biomass only during the earlier phase of the experiment, reaching values 4.7 times greater at the highest nutrient addition than at the normal loading rate. The structure of the phytoplankton community shifted from an initial dominance of picophytoplankton to dominance of microphytoplankton at the highest nutrient loadings Primary production increased in response to increased nutrient loading, reaching a level 10-fold higher at the highest nutrient loading than at the normal loading rate. However, phytoplankton carbon turnover did not increase significantly with increasing loading (p > 0.05), except for a tendency for higher turnover at the highest nutrient inputs tested. Results from this experiment suggest that eutrophication problems are likely to become important at a loading of more than 4-fold higher (20 mm01 N m" d-' and 1 rnrnol P m-' d-') than the present values supplied to the coastal Medlterranean community studied.

Episodic meteorological and nutrient-load events as drivers of coastal planktonic ecosystem dynamics: a time-series analysis

“…However, with a few days lag, biomass of autotrophic organisms responds to this increase in inorganic nutrient concentration. This lag is within the same range found by Duarte et al (2000) in a mesocosm experiment conducted in Blanes Bay during July. They found a lag of between 4 and 12 d in the response of chl a to nutrient additions, depending on the level of nutrient enrichment.…”

“…The negative correlations found with Secchi disk depths support this scenario. However, in the experimental setup in Duarte et al (2000), there was no increase in turbidity as the nutrients were added without extra additions. Thus, the delay could be explained entirely by a relatively slow response of phytoplankton.…”

“…In these oligotrophic conditions the microbial food web dominated, as suggested by the strong coupling between bacterial and HNF concentrations. The higher ammonium concentration indicates a preponderance of recycling processes over new production, or that ammonium is the dominant form of anthropogenic nitrogen input in summer in Blanes Bay (Duarte et al 2000).…”

“…In these oligotrophic conditions the microbial food web dominated, as suggested by the strong coupling between bacterial and HNF concentrations. The higher ammonium concentration indicates a preponderance of recycling processes over new production, or that ammonium is the dominant form of anthropogenic nitrogen input in summer in Blanes Bay (Duarte et al 2000).In the NW Mediterranean, high sea surface temperatures in summer are usually followed by heavy rainfall events in autumn (Llasat & Puigcerver 1997). Thus, the second part of the period was characterised by several heavy rainstorms.…”

The Impact of Atmospheric Dry Deposition Associated Microbes on the Southeastern Mediterranean Sea Surface Water following an Intense Dust Storm

“…This is consistent with the expected change derived from the PO 3− 4 addition (∼1.5 nM) and its subsequent carbon addition based on the C to chl-a ratio (see above). The relatively limited change in picophytoplankton abundance implies a possible top-down control of small-size autotrophs by grazers (Billen, 1990), dominance of larger-size algae such as microphytoplankton that utilized most of the leached nutrients/trace metals and compete with the picophytoplankton (Duarte et al, 2000), or toxicity of small-size autotrophs (Mann et al, 2002;Paytan et al, 2009). It may also hint at interactions between the ambient population and the airborne microbes that affect the abundance of small-size autotrophs.…”