Nuria Navarro

Control of air‐sea CO2 disequilibria in the subtropical NE Atlantic by planktonic metabolism under the ocean skin

The air-sea CO 2 gradient at the subtropical NE Atlantic was strongly dependent on the metabolism of the planktonic community within the top cms, but independent of that of the communities deeper in the water column. Gross primary production (GPP) and community respiration (R) of the planktonic community within the top cms exceeded those of the communities deeper in the water column by >10-fold and >7 fold, respectively. Net autotrophic metabolism (GPP > R) at the top cms of the water column in some stations drove CO 2 uptake by creating a CO 2 deficit at the ocean surface, while net heterotrophic metabolism (GPP < R) at the top cms of the water column in other stations resulted in strong CO 2 supersaturation, driving CO 2 emissions. These results suggest a strong control of the air-sea pCO 2 anomaly by intense biological processes.

Ecosystem metabolism and carbon fluxes of a tidally-dominated coastal lagoon

The metabolism and carbon flux in the western sector of the highly dynamic coastal lagoon Ria Formosa (south Portugal) were assessed to elucidate the relative importance of the contribution of the main communities, the treated sewage inputs from the adjacent city of Faro, and the exchange with the adjacent coastal waters to the ecosystem metabolism. The results depict the Ria Formosa as being a highly productive ecosystem dominated by the seagrassZostera noltii. The community dominated by the seagrassCymodocea nodosa had half of the gross production ofZ. noltii, followed by bare sediments and phytoplankton. The net contribution of seagrasses to community metabolism was negligible, as bothZ. noltii andC. nodosa showed a production: respiration ratio close to 1. Benthic microalgae emerge as the most important components of the net metabolism. The western sector of Ria Formosa was in metabolic balance during the summer when the study was done. Even though the total net ecosystem production was 7.22 Kmol C d−1, the error associated with this estimate was 8.38 Kmol C d−1, so ecosystem net production was not significantly different from zero. The Ria Formosa ecosystem is shallow and rapidly flushed by the tides, which force an important exchange of dissolved organic carbon (DOC) and particulate organic carbon (POC) with the adjacent coastal waters. The daily net export rate to the adjacent coastal waters, 0.98 Kmol d−1, represented 7.6% of the net ecosystem production, suggesting that the bulk of the net ecosystem production accumulates within the ecosystem. The organic carbon retention in the western sector of the Ria Formosa is higher than net production, because the allochthonous carbon inputs from urban sewage enter the carbon mass balance with about 40% of the autochthonous processes, at about 1.6 Kmol d−1 of DOC and 2.8 Kmol d−1 of POC. The western sector of Ria Formosa has an organic carbon sink of about 46.4 tons per year. Most of this is harvested in the form of molluscs (clams, cuttlefish, etc.) and fish (sea bream, sea bass, etc.). The total carbon harvested every year in the form of bivalves is about 40 tons, rendering the Ria Formosa the most productive seafood area in Portugal.

Use of freeze-dried microalgae for rearing gilthead seabream, Sparus aurata, larvae: I. Growth, histology and water quality

Abstract Four gilthead seabream larval feeding treatments [(A) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition to the larval tank of freeze-dried microalgae; (B) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition to the larval tank of live microalgae; (C) larvae fed rotifers cultivated with freeze-dried microalgae and without addition of microalgae to the larval tank; and (D) larvae fed rotifers cultivated with live microalgae and daily addition to the larval tank of live microalgae) were tested in order to determine if freeze-dried microalgae ( Nannochloropsis oculata ) was as good as live microalgae for larval rearing at two stages (for rotifer production and into larval tanks). The results showed that larvae can be reared successfully using only freeze-dried microalgae with a survival of ≈100% and without altering water quality. Fifteen-day-old larvae were obtained with normal growth in length (5.56±0.14 mm) and weight (104.46±1.86 μg) and with good development of the digestive tract and other associated organs (liver, pancreas). The presence of microalgae (freeze-dried as well as live) in the rearing tanks significantly influenced growth, survival and histological status.

Feeding behaviour of the rotifers Brachionus plicatilis and Brachionus rotundiformis with two types of food: live and freeze-dried microalgae

Abstract Freeze-dried cells are accepted and ingested by Brachionus plicatilis and Brachionus rotundiformis in the same way as live cells. B. plicatilis can daily ingest 60–90% of their dry weight, while B. rotundiformis can daily ingest 160% of their dry weight. In B. plicatilis , the relationship between cell concentration and filtration and ingestion rates follows the criterion of energy maximization. As food concentration increases from very low values, both rates rise until an inflexion point is reached. Above this point, the maximum food processing rate could be maintained with filtration rates that decrease progressively, and with ingestion rates that maintain the maximum value reached. In B. rotundiformis , filtration rates decrease steadily while ingestion rates increase rapidly as food concentration increases. Once an inflexion point is reached, further increases in food processing are very slow. Experiment duration is also a factor which significantly influences feeding rates, so that after the first hour filtration and ingestion rates are lower and remain almost constant.

Population dynamics of rotifers (Brachionus plicatilis and Brachionus rotundiformis) in semicontinuous culture fed freeze-dried microalgae: influence of dilution rate

Abstract Freeze-dried microalgae can be used in establishing semicontinuous cultures for rotifers with high efficiency. The populations reach a quasi-steady state as the result of a process of regulation by means of exchanging material and energy between the population and their food, giving that at the beginning of each dilution, rotifer density is always the same, since population growth rate balances dilution losses. At this state, the specific growth rate (r) is constant and independent of time. The control of the semicontinuous production is obtained by the dilution rate (D). Several dilution rates were tested and those that resulted in the best productions (mg rotifer day−1) and food conversion efficiencies (mg rotifer developed per mg microalgae consumed) were D=0.3 day−1 for Brachionus plicatilis and D=0.2 day−1 for Brachionus rotundiformis.

Use of freeze-dried microalgae for rearing gilthead seabream, Sparus aurata L., larvae. II. Biochemical composition

The objectives of this study were to test whether freeze-dried microalgae are nutritionally adequate for rearing rotifers as food for gilthead seabream larvae. The elemental composition (C, N, H) and fatty acid composition were analysed in larvae of gilthead seabream, Sparus aurata L., rotifers Brachionus plicatilis and Brachionus rotundiformis and freeze-dried microalgae Nannochloropsis oculata. Four larval feeding treatments were tested: (A) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition of freeze-dried microalgae to the larval tanks; (B) larvae fed rotifers cultivated with freeze-dried microalgae and daily addition of live microalgae to the larval tanks; (C) larvae fed rotifers cultivated with freeze-dried microalgae, without addition of microalgae to the larval tank and (D) larvae fed rotifers cultivated with live microalgae and daily addition of live microalgae to the larval tanks. No significant differences were observed between the biochemical composition of larvae with treatment A (with freeze-dried microalgae) and the composition of larvae in treatment D that were obtained with the acceptable methods for culture systems (with live microalgae).

Temperature dependence of plankton community metabolism in the subtropical and tropical oceans: Temperature Dependence of Subtropical Plankton Metabolism

Here we assess the temperature dependence of the metabolic rates (gross primary production - GPP, community respiration - CR and the ratio GPP/CR) of oceanic plankton communities. We compile data from 133 stations of the Malaspina 2010 Expedition, distributed among the subtropical and tropical Atlantic, Pacific and Indian oceans. We used the in vitro technique to measured metabolic rates during 24 h incubations at three different sampled depths: surface, 20% and 1% of the photosynthetically active radiation measured at surface. We also measured the % of ultraviolet B radiation (UVB) penetrating at surface waters. GPP and CR rates increased with warming, albeit different responses were observed for each sampled depth. The overall GPP/CR ratio declined with warming. Higher activation energies (Ea´s) were derived for both processes (GPPChla = 0.97; CRChla = 1.26; CRHPA= 0.95 eV) compared to those previously reported. The Indian Ocean showed the highest Ea (GPPChla = 1.70; CRChla = 1.48; CRHPA= 0.57 eV), while the Atlantic Ocean showed the lowest (GPPChla = 0.86; CRChla = 0.77; CRHPA= -0.13 eV). We believe that the difference between previous assessments and the ones presented here can be explained by the overrepresentation of Atlantic communities in the previous data sets. We found that UVB radiation also affects the temperature dependence of surface GPP, which decreased rather than increased under high levels of UVB. Ocean warming, which causes stratification and oligotrophication of the subtropical and tropical oceans, may lead to reduced surface GPP as a result of increased penetration of UVB radiation.