Grazing rate of microzooplankton measured experimentally in the North Atlantic during the Maria S. Merian cruise MSM26, spring 2013

The microzooplankton grazing dilution experiments were conducted at stations 126, 127, 131 and 133-137, following Landry & Hassett (1982). Seawater samples (whole seawater - WSW) were taken via Niskin bottles mounted on to a CTD Rosette out of the chlorophyll maximum at each station. Four different dilution levels were prepared with WSW and GF/F filtered seawater - 100% WSW, 75% WSW, 50% WSW and 25% WSW. The diluted WSW was filled in 2.4 L polycarbonate bottles (two replicates for every dilution level). Three subsamples (250 - 500 mL depending on in situ chlorophyll) of the 100% WSW were filtered on to GF/F filters (25 mm diameter) and chlorophyll was extracted in 5 mL 96% ethanol for 12-24 hours. Afterwards it was measured fluorometrically before and after the addition of HCl with a Turner fluorometer according to Jespersen and Christoffersen (1987) on board of the ship. In addition, one 250 mL subsample of the 100% WSW was fixed in 2% Lugol (final concentration), to determine the microzooplankton community when back at the Institute for Hydrobiology and Fisheries Science in Hamburg. Also, one 50 mL subsample of the 100% WSW was fixed in 1 mL glutaraldehyde, to quantify bacteria abundance. The 2.4 L bottles were put in black mesh-bags, which reduced incoming radiation to approximately 50% (to minimize chlorophyll bleaching). The bottles were incubated for 24 hours in a tank on deck with flow-through water, to maintain in situ temperature. An additional experiment was carried out to test the effect of temperature on microzooplankton grazing in darkness. Therefore, 100% WSW was incubated in the deck tank and in two temperature control rooms of 5 and 15°C in darkness (two bottles each). The same was done with bottles where copepods were added (five copepods of Calanus finmarchicus in each bottle; males and females were randomly picked and divided onto the bottles). In addition, two 100% WSW bottles with five copepods each were incubated at in situ temperature at 100% light level (without mesh-bags). All experiments were incubated for 24 hours and afterwards two subsamples of each bottle were filtered on to GF/F filters (25 mm diameter); 500 - 1000 mL depending on in situ chlorophyll. One 250 mL subsample of one of the two replicates of each dilution level and each additional experiment (temperature and temperature/copepods) was fixed in 5 mL lugol for microzooplankton determination. One 50 mL subsample of one of the two 100% WSW bottles as well as of one of the additional experiments without copepods was fixed in 1 mL glutaraldehyde for bacteria determination later on. Copepods were fixed in 4% formaldehyde for length measurements and sex determination.

Variación de la concentración de nitratos en un suelo franco limoso del Alto Valle de Río Negro.

La abundancia y bajo costo del recurso hídrico en el Alto Valle de Río Negro combinados con un manejo ineficiente del mismo, principalmente durante la primera parte de la primavera, época en la que los productores riegan con mayor frecuencia para luchar pasivamente contra las probables heladas tardías, permiten inferir que los nitratos presentes en el suelo, así como el aportado por los fertilizantes nitrogenados, están sujetos al lixiviado durante una gran parte del ciclo productivo. En la actualidad no existen estudios regionales que ilustren la variación estacional de la concentración de nitratos en la zona de exploración radical de frutales, por lo que se inició el presente trabajo con el propósito de: a) medir la concentración de los nitratos en el perfil del suelo cultivado con manzanos, desde el período de floración hasta el inicio de caída de hojas, con fertilización nitrogenada en dos dosis y sin fertilización a distintas profundidades de extracción; b) determinar la eficiencia del riego a manto de dicho monte. Se ensayaron dos concentraciones de nitrógeno, adicionado como nitrato de amonio en dos oportunidades: el 50% a la caída de los pétalos y el 50% restante cercano a la cosecha, correspondiendo a dosis de 100 kg ha-1 (N1), 200 kg ha-1 (N2) y un testigo sin agregado de N (N0), durante el período 2004-2005 y 2005-2006. Para determinar los niveles de N en el suelo, expresado como nitratos, se extrajeron muestras del mismo a tres profundidades 0-30; 30-60; 60-90 cm, al inicio de floración, antes del primer riego y después de cada riego. La lámina de agua empleada para el riego a manto osciló entre 1712 y 2400 mm, con un aprovechamiento a campo del 30%. La concentración de nitratos fue baja cuando no se fertilizó, manteniéndose alrededor de 22 mg kg-1 en superficie y reduciéndose a la mitad a la profundidad de 30-60 cm, durante el período de muestreo. En ambas dosis empleadas, el contenido de nitratos del suelo fue mayor llegando a 175 y 300 mg kg-1, respectivamente. Estos valores se igualan a los del testigo a los 30 días en el caso de N1 y a los 60 días para N2. Los resultados permiten inferir que la concentración de nitratos fue efímera en el perfil del suelo y mejoró la eficiencia de riego, principalmente durante la primavera con el fin de minimizar pérdidas de nitrógeno.

(Table 2) Concentration of phytoplankton pigments in Adventfjorden in spring and summer of 1997

The study was carried out from April 30 until July 13 of 1997 in Adventfjorden (Spitsbergen). Formation of a less saline and warmer surface water (~1 m thick) caused by melting of the ice was observed in the fjord during the first days of May. In summer the less saline surface layer was about 3 m thick. Euphotic depth measured under the ice sheet reached 12 m, whereas load of mineral matter brought with riverine discharge in summer (content of total particulate matter in the fjord reached 1.66 kg/m**2) dramatically reduced euphotic zone depth to 0.35 m. By pigment measurement three phases of phytoplankton development in Adventfjorden were distinguished: (1) spring bloom that has started under fast ice and reached maximum in the mid of May, (2) stagnation period in June, (3) increase of pigment concentration in July, what could indicate start of the next algae bloom. Analyses of chlorophylls and carotenoids revealed that diatoms (chl c, fucoxanthin), and green algae (chl b, lutein) dominated phytoplankton community in the fjord. Moreover, presence of peridinin indicates presence of Dinophyta and alloxanthin - occurence of Cryptophyta. In May and June 1997 phytoplankton appeared mainly in the surface of water, while in July, as a result of inflow of turbulent riverine waters into Adventfjorden, algae cells were pushed down and the highest numbers were observed at depth ~20 m. Great phaeopigments to chl a ratio (= 0.54) found in fjord seston in June and July probably shows strong impact of zooplankton grazing on phytoplankton development. High contribution of chlorophyllide a in porphyrin a poll in samples collected under fast ice (chlorophyllide a / chl a ratio = 0.18) reflects the final stage of algal communitie succession in ice, just before spring ice melt and release of biota to oceanic water. Chlorophyllide a content during summer was minor or not detectable, demonstrating that diatom cells were in good physiological condition. High chl a allomer / chl a ratio (average = 0.11 for the period investigated) confirms high oxygen concentration in environment of Adventfjorden.