Metabolism of mesozooplankton across the Benguela upwelling system in terms of ETS and GDH activities

Northern Benguela upwelling: Driving forces and ecosystem responses I. Fernández-Urruzola, A. Herrera, L. Postel, N. Osma, M. Gómez and T. T. Packard etabolism of mesozooplankton across the Benguela upwelling system in terms of ETS and GDH activities M Institute of Oceanography and Global Change, Ecophysiology of Plankton Group, University of Las Palmas de Gran Canaria, Canary Islands, Spain. E-mail address: [email protected] INTRODUCTION Respiration in the ocean ecosystems Respiration is a ubiquitous process which constitutes a key component in the estimation of the carbon flux. However, its magnitude in ocean ecosystems remains uncertain (Del Giorgio and Duarte, 2002) due to the difficulty on quantifying in situ respiration rates. They provide low data acquisition rates and are complicated by organisms manipulation, overcrowding and starvation (Bidigare, 1983). WATER-BOTTLE INCUBATIONS Potential measurements such as ETS avoid these methodological artifacts!! (I) NH4+ is an intermeditate product in decomposition of organic matter, which constitutes the most reduced form of N. (II) NH4+ sustains a global average of 80 % of the authotroph’s requeriments (Harrison, 1992), with the mesozooplankton responsible for 12 – 23 % (Hernández-León, 2008). Zehr and Kudela (2011) INTRODUCTION Importance of NH4+ in marine systems RECYCLYING EFFICIENCY Nutritional History (Miller and Roman, 2008; Saba et al., 2009) Temperature (Ikeda, 1985) Trophic Interactions (Glibert, 1998) INTRODUCTION Electron Transport System as index for O2 respiration Vmax INTRODUCTION Glutamate dehydrogenase as index for NH4+ respiration GDH (EC 1.4.1.3) is found in high levels in planktonic crustaceans (Regnault, 1987). Its role in amino acids catabolism agues for its control over a great proportion of NH4+ excretion. Modified from Yuen and Chiew (2010) INTRODUCTION Benguela Upwelling System Namibian coast is characterized by intense coastal upwelling associated with Benguela current. These upwelling areas provide high organic production linked to phytoplankton bloom. Map of sea surface temperature from the MODIS sensor on board NASA Aqua satellite (3 Feb.2008). We estimated biomass, as well as the potential respiration (ETS) and NH4+ excretion (GDH) in mesozooplankton community in three depths along a transect with different oceanographic conditions to understand how affect phytoplankton bloom in the zooplankton metabolism. MATERIAL AND METHODS Study area 4 sections across the northern Benguela upwelling system: LT1,LT2 and LT3 with 12 stations, and LT4 with 21 stations. '-:::~- --:-- -- . -- _. 1-·· 1\ . --_ . . -- _. - -...,_ . "."';'" . -_ .. _. ..... ... _. _ . \ MATERIAL AND METHODS Experimental design SIZE FRACTIONATION Data analysis Laboratory work at institute TAXONOMY, BIOMASS and METABOLISM More than 700 samples were analyzed for metabolic rates Storage in criovials at -80 oC GDH Activity (Bidigare and King, 1981) ETS Activity (Packard et al., 1971) Protein mass (Lowry, 1951) 0 50 100 150 200 250 18:00 18:14 18:28 18:43 18:57 19:12 19:26 Tiefe [m] 457N070301, 13-9-2000, 200-75 und 75-25m ZOOPLANKTON SAMPLING 100-200 μm 200-500 μm 500-1000 μm > 1000 μm RESULTS Global averages from the different sections The higher biomass values are coupled to the upwelled waters. ETS activity fits with the biomass pattern, while GDH activity present more dispersion since fitoplancton does not have effect in this rate. However, the GDH maximum rate is still next to the upwelling. Biomass [mg protein . (m3)-1] NAM027 NAM025 NAM023 NAM021 NAM019 NAM017 NAM014 NAM009 NAM004 NAM001 r-- __ _ • • • • • 4 • 3 50 • • • • • • • E' '-'- 5 ~ 2 Q 100 1 • • • 150 O 500 400 300 200 100 O Section Distance [km] RESULTS Biomass and enzyme activities for each “short” section SECTION 1 There is a bloom on NAM007, which causes high enzimatic rates in this area. This active metabolism seems to move offshore along the subsuperficial waters, specially for GDH. SECTIONS 3 and 4 These two sections show a similar pattern than that found in section one. The bloom is now placed around NAM005 and it slightly extends offshore. SECTION 2 Biomass and ETS activity are mostly distributed in all the water column of NAM009. The latter, however, presents its maximum between 200-75 m. This not seems to affect GDH, whose higher activity is in the surface waters of NAM017 probably due to a peak of zooplankton in this region RESULTS Size fraction contribution to the mean values in terms of Biomass 100-200 μm 200-500 μm 500-1000 μm > 1000 μm All the fractions present high values onshore. However, the lower the size fraction is, the more coupled is with the upwelling. Consequently, the mesozooplankton over 500 μm seems to dominate offshore, with maximum values on NAM023 for the highest fraction. RESULTS Size fraction contribution to the mean values in terms of ETS FRACCIONES INTEGRADAS ETS 100-200 μm 200-500 μm 500-1000 μm > 1000 μm All the fractions present high values onshore. However, the lower the size fraction is, the more coupled is with the upwelling. Consequently, the mesozooplankton over 500 μm seems to dominate offshore, with maximum values on NAM023 for the highest fraction. RESULTS Size fraction contribution to the mean values in terms of GDH FRACCIONES INTEGRADAS GDH 100-200 μm 200-500 μm 500-1000 μm > 1000 μm All the fractions present high values onshore. However, the lower the size fraction is, the more coupled is with the upwelling. Consequently, the mesozooplankton over 500 μm seems to dominate offshore, with maximum values on NAM023 for the highest fraction. RESULTS What does the statisctics say? 1. Are there significant differences in ETS, GDH and Biomass between DEPTHS? YES!! (Kruskal-Wallis test, p<0.001) RESULTS What does the statisctics say? 2. Are there significant differences in ETS, GDH and Biomass between SIZE FRACTIONS? YES!! (Kruskal-Wallis test, p<0.001) RESULTS What does the statisctics say? YES!! (Mann-Whitney test, p<0.001) 3. Are there significant differences in ETS, GDH and Biomass between UPWELLED and OFFSHORE WATERS? RESULTS What does the statisctics say? 4. Are there significant differences in ETS, GDH and Biomass between DAY/NIGHT SAMPLING? NO!! (Mann-Whitney test, p>0.001) ✗ RESULTS Metabolism and biomass relationship: Kleiber’s law For many years it has been accepted the paradigm of the Kleiber´s law, which stablish a exponential relationship between biomass and metabolic rates with a coefficient “b” close to 0.75. Several zooplankton studies show different ratios close to or greater than 0.75 in optimal feeding conditions, and below 0.75 when the organisms are found in oligotrophic areas or poor feeding (Gómez et al., 2008; Herrera et al., 2011; Martínez, 2007; Packard and Gómez, 2008). In this study, the coefficients “b” are greater than 0.75 for ETS and GDH activities in both upwelling and offshore areas. Metabolic rate = aWb (W=biomass) RESULTS Metabolism and biomass relationship: Kleiber’s law OFFSHORE log ETS = 1.05 log W + 0.68 UPWELLING log ETS = 1.08 log W + 0.67 ETS vs Biomass