Mangroves and Salt Marshes 3: 185–195, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 185 Nutrient dynamics in a mangrove creek (North Brazil) during the dry season R. J. Lara ∗ & T. Dittmar Zentrum für Marine Tropenökologie, Fahrenheitstr. 1, 28359 Bremen, Germany; ∗ Address for correspondence: Laboratorio de Ciências Ambientais, Projeto MADAM (UFPa/NUMA), Avda Perimetral 2651, Terra Firme, CEP: 66077-530, Bel´ em, Par´ a, Brazil; E-mail: belmadam@supridad.com.br (Received 25 February 1999; accepted in revised form 30 April 1999) Key words: groundwater, hydrology, primary producers, tides, variability Abstract During the dry season, dissolved inorganic nutrients and organic carbon (DOC) in a mangrove tidal creek (Bragança, North Brazil) presented a clear tidal signature: silicate, phosphate, ammonium and DOC covaried with salinity, with minima at high tide and maxima at low tide. Hydrodynamical calculations explained most diel variability as dilution of nutrient-rich groundwater input by estuarine water. Creek nutrients tended to lower concentrations towards the end of the dry season, probably due to changes in groundwater dynamics. Ammonium was about 44% higher in the night than during the day, while silicate, phosphate nitrate + nitrite showed differences < 5%, indicating preferential autotrophic uptake of ammonium as nitrogen source. Nitrification and/or nitrogen fixation are probably sources of nitrate + nitrite, which contributed only 12% to the dissolved inorganic nitrogen pool. Dissolved oxygen (7%) and DOC (11%) were lower during the night, suggesting the existence of a labile, algal-derived DOC pool. There is a loose coupling between aquatic primary production, groundwater dynamics and tidal regime. Introduction Do mangroves represent a significant source of inor- ganic and organic nutrients for adjacent coastal waters or, alternatively, function as a sink for carbon and nutrients (Boto and Wellington, 1988; Alongi et al., 1989; Boto and Robertson, 1990)? The accurate de- termination of material fluxes is often prevented by large and apparently random tidal-dependent oscilla- tions (Boto and Wellington, 1988). The knowledge of the driving forces behind these variations may help clarify actual transport dynamics. Within the man- groves, nutrients can be transported from primary sources to consumers by several mechanisms, includ- ing movements of the water table, capillarity, tides, etc. Among others, river runoff, groundwater input and nitrogen fixation are processes which may con- stitute primary sources of nutrients for the mangrove itself. Comparison of flux studies from different re- gions presents difficulties due to the different char- acteristics of the various mangrove environments. In general, three main factors are responsible for the ma- jor differences: terrestrial runoff, topography and tidal range. Tidal amplitudes along the Brazilian coast vary from about 1–5m, offering a wide range of envir- onmental conditions under which mangroves can de- velop. Although Brazilian mangroves are among the world’s largest, very little is known about their nu- trient dynamics. The available information is mostly referred to the microtidal region (e.g. Ovalle et al., 1990). In North Brazil macrotides are of such charac- teristics that they constitute the main hydrodynamical feature of mangroves. Amplitudes can reach about 4 m and current velocities may be as high as 1.5 m/s during spring tides (Cohen et al., 1998). Clearly, tidal mo- tion has to be considered as a possible main vehicle for nutrient transport. In this area, mangrove forests are completely flooded only fortnightly during spring tides. These features, and the presence of numerous