Small time scale plankton structure variations at the entrance of a tropical eutrophic bay (Guanabara Bay, Brazil)

BRAZILIAN JOURNAL OF OCEANOGRAPHY, 60(4):405-414, 2012 SMALL TIME SCALE PLANKTON STRUCTURE VARIATIONS AT THE ENTRANCE OF A TROPICAL EUTROPHIC BAY (GUANABARA BAY, BRAZIL) Mariana Guenther1,3*, Isabel Lima1, Glenda Mugrabe1, Denise Rivera Tenenbaum1, Eliane Gonzalez-Rodriguez2 and Jean Louis Valentin1 1 Universidade Federal do Rio de Janeiro - Departamento de Biologia Marinha, Instituto de Biologia (21949-900 Rio de Janeiro, RJ, Brasil) 2 Instituto de Estudos do Mar Almirante Paulo Moreira - Departamento de Oceanografia (28930-000 Arraial do Cabo, RJ, Brasil) 3 Present Address: Universidade de Pernambuco - Instituto de Ciências Biológicas (50100-130 Recife, PE, Brasil) *Corresponding author: ABSTRACT The dynamics of the plankton compartments at the entrance of Guanabara Bay (SE Brazil) were assessed during a short-term temporal survey to estimate their trophic correlations. Size-fractioned phytoplankton (picoplankton: < 2µm, nanoplankton: 2-20µm and microplankton: > 20µm) biomass and photosynthetic efficiency, composition and abundance of the auto- and heterotrophic nano- and microplankton, and mesozooplankton were evaluated at a fixed station for 3 consecutive days at 3-h intervals, in the surface and bottom (20m) layers. The variability of almost all plankton compartments in the surface layer was directly dependent on temperature, indicating the great influence of the circulation at the entrance of the bay on plankton structure. In the surface layer, the mesozooplankton seems to be sustained by both autotrophic nano- and picoplankton, this last being channeled through the microzooplankton. Near the bottom, both auto- and heterotrophic microplankton are probably supporting the mesozooplankton biomass. Our findings thus suggest that the entrance of Guanabara bay presents a multivorous food web, i.e., a combination of both grazing and microbial trophic pathways. RESUMO A dinâmica dos vários compartimentos do plâncton foi avaliada durante uma série de curta duração na entrada da baía de Guanabara (SE do Brasil), com o objetivo de estimar suas correlações tróficas. A biomassa e eficiência fotossíntética das três frações do fitoplâncton (picoplâncton: < 2µm, nanoplâncton: 2-20µm e microplâncton: > 20µm), juntamente com a composição e abundância do nano- e microplâncton auto- e heterótrofos e do mesozooplâncton, foram determinadas em uma estação fixa durante 3 dias consecutivos, a intervalos de 3h, nas camadas de superfície e de fundo (20m). A variabilidade de quase todos os compartimentos do plâncton na superfície foi diretamente relacionada à temperatura, indicando forte influência da circulação da entrada da baía na estrutura planctônica. Na camada superficial, o mesozooplâncton parece ser alimentado pelo nano- e picoplâncton autótrofos, esse último sendo sustentado pelo microzooplâncton. Próximo ao fundo, o microplâncton auto- e heterótrofo estão possivelmente sustentando a biomassa mesozooplanctônica. Nossos resultados sugerem, portanto, que na entrada da baía de Guanabara esteja estabelecida uma rede trófica multívora, i.e., uma combinação entre as cadeias microbiana e de pastagem. Descriptors: Phytoplankton, Microzooplankton, Mesozooplankton, Grazing food web, Microbial food web, Carbon fluxes. Descritores: Fitoplâncton, Microzooplâncton, Mesozooplâncton, Rede trófica de pastagem, Rede trófica microbiana, Fluxos de carbono. INTRODUCTION The comprehension of the plankton community structure of a system is crucial for predicting the carbon fluxes within its food webs and determining its export processes (e.g. TIAN et al., 2000; RIVKIN; LEGENDRE, 2002; CALBET; LANDRY, 2004; VARGAS et al., 2007; STUKEL et al., 2011). Generally, the plankton trophic structure of temporally steady aquatic systems, such as lakes and open oceans, is usually related to the water column stability and nutritional profile: in more eutrophic and turbulent systems, larger phytoplankton cells prevail, leading to the establishment of the shorter classical (or grazing) food web, while in more oligotrophic and stratified systems the smaller producers dominate the plankton, 406 BRAZILIAN JOURNAL OF OCEANOGRAPHY, 60(4), 2012 and the longer microbial food web prevails (LEGENDRE; LE FEVRE, 1995; LEGENDRE; RASSOULZADEGAN, 1996). In estuaries and coastal bays, the lateral transport and pulses of nutrients and organisms from both terrestrial and oceanic sides result in a hydrodynamic complexity such that the short term variations in chemical and physical parameters like temperature, salinity, nutrient and organic matter contents and tidal currents have an important impact on the biotic interactions (DYER, 1997; MIRANDA et al., 2002). It is thus to be expected that the trophic relationships within the plankton in these systems will be more complex than those stated for steadier ones. Guanabara Bay (22°41’- 22°56’S; 43°02’43°18’W) has a tropical humid climate (Aw) with dry cool winters and wet warm summers (KÖPPEN, 1900). This system presents a wide spatial eutrophication gradient, increasing from the entrance to the inner bay (MAYR et al., 1989). This variation is due to the balance between terrestrial influence, i.e., inputs of high loads of nutrients and organic material from riverine sources and from domestic and industrial sewage, and oceanic forces, i.e., strong tidal currents (80 – 150 cm s-1) at the narrow entrance of the bay (JICA, 1994). The plankton communities of Guanabara Bay have been well studied since the beginning of the XXth century on both short and long term scales, providing a relevant array of information about the dynamics of the autotrophic microplankton (VILLAC; TENENBAUM, 2010 and references therein), bacterioplankton (GUENTHER et al., 2008b), auto- and heterotrophic nanoplankton (SANTOS et al., 2007), microzooplankton (GOMES et al., 2007) and mesozooplankton (e.g. WANDERNESS et al., 1997; VALENTIN et al., 1999; GOMES et al., 2004; SCHWAMBORN et al., 2004). Nevertheless, none of them has yet evaluated the dynamics of the auto- and heterotrophic plankton compartments simultaneously. The present study was undertaken at the entrance of Guanabara Bay during the summer (= wet season) over a short time scale, covering the end of the spring tide and the beginning of the neap tide. The variability of the physical and chemical conditions of this site and period, discussed in greater detail in a previous paper (GUENTHER et al., 2008b), indicates a highly hydrological complexity. During the spring tide, there was great water-column mixing with high homogeneity of temperature, salinity and nutrients over depth. However, at the neap tide, the high watercolumn stability suggested the establishment of gravitational circulation, with outflow at the surface and inflow near the bottom. This pattern was also corroborated by the increase in temperature and decrease in salinity at the surface, indicating the influence of the inner bay waters, and the decrease in temperature and increase in N- (...truncated)