compared with a preceeding study in which the same bell jar technique has been applied in the Sylt-Rømø Bay of the northern Wadden Sea. Nitrate flux was ...
Hydrobiologia 436: 217–235, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Nutrient fluxes in intertidal communities of a South European lagoon (Ria Formosa) – similarities and differences with a northern Wadden Sea bay (Sylt-Rømø Bay) Ragnhild M. Asmus1 , Martin Sprung2 & Harald Asmus1 1 Alfred-Wegener-Institut für 2 CCMAR-UCTRA,
Polar-und Meeresforschung, Wadden Sea Station Sylt 25992 List/Sylt, Germany Universidade do Algarve, Campus de Gambelas, P-8000810 Faro, Portugal
Received 28 May 1999; in revised form 3 July 2000; accepted 7 August 2000
Key words: nutrient flux, nitrogen, phosphate, tidal flats, temperature, geographical comparison
Abstract During an annual cycle, flux rates of oxygen, nitrate, nitrite, ammonium, phosphate and silicate were measured in light and dark bell jars at three sites in Ria Formosa (Algarve, Portugal) enclosing either a natural macrophytic community (macroalgae on sand or mud, a seagrass bed of Zostera noltii) or bare sediments. The results are compared with a preceeding study in which the same bell jar technique has been applied in the Sylt-Rømø Bay of the northern Wadden Sea. Nitrate flux was mainly directed from the water column to the benthic communities in Ria Formosa, as well as in the Sylt-Rømø Bay. However, nitrate uptake was higher in the northern, more eutrophic study area. In Ria Formosa, nutrient concentrations were lower than in the Sylt-Rømø Bay possibly due to strong water exchange with Atlantic waters. High temperatures and strong insolation had a greater impact on nitrate fluxes in Ria Formosa than in the Sylt-Rømø Bay. Bioturbating macrofauna increased ammonium efflux in the Sylt- Rømø Bay while this effect was not as pronounced in the Ria Formosa study sites. Benthic phosphate uptake dominated in the Ria Formosa and was correlated to initial phosphate concentrations in incoming waters. At both study sites, oxygen and nutrient fluxes were correlated with temperature. Additionally, flux rates were strongly influenced by biotic components and levels of eutrophication. A literature survey showed that mainly in temperate regions, material fluxes increase with temperature, whereas in warmer areas, ammonium and phosphate fluxes between sediment and water were generally lower. Introduction The nutrient and oxygen exchange between sediments and water has been studied extensively in the temperate regions of the United States and Europe since the pioneering work of Pamatmat (1968), Rowe et al. (1975) and Nixon et al. (1976). Despite differences between ecosystems, some common features emerge. Interactions of chemical, biological and physical processes determine the extent to which nutrients are released or taken up by benthic communities. Remineralisation increases with the amount of labile organic matter (Zeitzschel, 1980; Jensen et al., 1990, Sloth et al., 1995), and benthic-pelagic fluxes are dependent on the nutrient concentrations in the water column (Boynton et al., 1980; Asmus, 1986). Additionally,
the state of oxidation of sediments is important for the influx or efflux of nutrients (Rysgaard et al., 1996). Hence, eutrophication is of great importance (Nowicki & Oviatt, 1990). Benthic communities may alter exchange processes profoundly by bioturbation, excretion (Kristensen, 1984; Hüttel, 1990), as well as sediment enrichment with organic material by macrofauna (Baudinet et al., 1990). At least temporarily, retention of nutrients in the benthic system is caused by benthic microalgae (Granéli & Sundbäck, 1985; Rizzo, 1990), seagrasses and macroalgae (Sand-Jensen & Borum, 1991; Duarte, 1995). Bacterial remineralisation may be the source of nutrient release from sediments including denitrification (Seitzinger, 1987), but bacteria need inorganic nutrients as well and may act as nutrient sink (Alongi, 1991; Van Duyl et al., 1993; Zweifel
218 et al., 1993). The annual temperature regimes in study areas are of great importance to nutrient fluxes (Nixon et al., 1976). Hydrodynamics are acting via currents, waves and turbulence (Rutgers Van der Loeff, 1981; Wildish & Kristmanson, 1997; Asmus et al., 1998; Widdows et al., 1998). This may cause different results from mesocosms and flumes showing higher rates than in benthic chamber and core incubations (Asmus et al., 1998). In this study, data from Ria Formosa (Southern Portugal) are compared to previously published data from Sylt-Rømø Bay (Northern German Wadden Sea) using the same bell jar technique. Both ecosystems are quite similar under many aspects, but separated by 18◦ latitude (or 2350 km on a SW–NE axis). Main structural difference are extensive salt marshes in the Ria, not present in the Sylt-Rømø Bay. The two tidal basins have little freshwater input but a large water exchange by tidal inlets with coastal waters either of the Atlantic or of the North Sea. Hence, they are well mixed and oxygenated marine systems. Will the community metabolism be possibly higher caused by elevated temperatures in the southern ecosystem or will other processes dominate? Study sites Ria Formosa Ria Formosa is a tidal lagoon covering an area of 100 km2 at the southern coast of Portugal (Algarve). It is separated from the Atlantic Ocean by several sandy islands. Lagunal waters are exchanged with the Atlantic by 6 deep inlets. The tidal range varies between 2.8 m during spring tide and 0.6 m at neap tide. The residence time is extremely short varying between half a day and 2 days (Neves et al., 1996). Approximately two thirds of the area is intertidal during mean low tide, one third of the total lagoon area is occupied by saltmarshes or salinas (Teixeira & Alvin, 1978). Sediments are mainly muddy along the mainland coast and become more sandy close to the dune islands. Perennial seagrass beds extend subtidally (mainly Cymodocea nodosa (Ucria) Aschers., but also Zostera marina L. and Z. noltii Hornem.) and intertidally (Zostera noltii). In the inner part of the lagoon, green algae, mainly Ulva spp. and Enteromorpha spp., but also Fucus spp. and Gracilaria spp. regularly form thick mats in winter (up to 2970 g DW m−2 ; Reis & Sprung, 1995) which decay during summer (except Fucus). On the sand flats, seasonality is
Figure 1. Study sites in the Ria Formosa (Algarve, Portugal) in a muddy area (M), a sand flat (S) and a seagrass bed of Zostera noltii (SG) growing on muddy sand. The Königshafen Bay (arrow) is situated 2350 km northeast in the Sylt-Rømø tidal basin, a part of the Wadden Sea.
more erratic. Salinity is between 36 and 38 all year round apart from an occasional decrease after heavy rainfall in winter (Falcão et al., 1985). Nutrient flux measurements were carried out at a muddy station (M), a sandy site (S) and in a muddy Zostera noltii bed (SG) in the western part of Ria Formosa (Figure 1). Organic matter content of dried sediments ranged from 1 to 1.4% for the sandy sediment, for muddy sediments of the seagrass bed and under macroalgal covers from 7 to 8% with little seasonal variation (loss on ignition at 450 ◦ C for 3 h). Sylt-Rømø Bay In the North Sea, the Wadden Sea extends along the coast of the Netherlands up to southern Denmark. At the German-Danish border, the Sylt-Rømø Bay of 404 km2 is enclosed by the islands Sylt and Rømø and
219 2 dams connecting these islands with the mainland. Water is exchanged between the North Sea and the Sylt-Rømø Bay through one large tidal gully. The tidal range is 1.80 m, and salinity varies seasonally between 28 and 32. The residence time in the Sylt-Rømø Bay is about 51 days (Müller, unpublished). One third of the Sylt-Rømø area is intertidal, only small salt marshes exist. The study site ‘Königshafen’ is a small part (5.5 km2) of the total bay at the northern tip of the island of Sylt. Main types of benthic communities of this Wadden Sea area are sand flats, mussel beds, small mud flats and seagrass beds (Zostera noltii, Z. marina). Organic content of sandy sediments ranges from 0.5 to 1.0% and from 2 to 6% for muddy sediments (loss on ignition) (Kristensen et al., 1997). Macroalgae, mainly green algae, only thrive during particular summers to a significant extent (Reise et al., 1989).
intensity was monitored by a discoidal sensor in air by a Li-Cor recorder. In the Sylt-Rømø Bay, flux rates of oxygen, nitrate, nitrite, ammonium and silicate were monitored monthly during an annual cycle in 1980. A pair of light and dark bell jars each were incubated on an Arenicola sand flat, a Nereis sand flat and a sandy Zostera noltii seagrass bed. The results from the bell jar measurements in the Sylt-Rømø Bay have already been published (Asmus, 1986) and serve as comparison to the results obtained in Ria Formosa. In addition, nutrient fluxes and primary production were measured in core incubations in 1993/94 in Königshafen at the same sites showing similar nitrogen fluxes as some 10 years ago, but significantly higher oxygen production in the 1990s than in the 1980s by microphytobenthos (Kristensen et al., 1997; Asmus et al., 1998a, b). Statistics
Materials and methods Bell jar technique Light (plexiglas) and dark bell jars (PVC, area 650 cm2 ) were pushed into the sediment just before the tide reached the study site. During rising tide, the bell jars were filled with water (volume 17.7 l). Wave action was transmitted into the bell jars by the flexible plastic foils covering their top. In the Sylt-Rømø Bay, wave action was strong enough to secure water mixing inside the bell jars, in Ria Formosa, an additional hand driven stirrer was inserted. The study sites in Ria Formosa were monitored every second month during an annual cycle (1990–1991). On the sand (S) and mud flats (M), one pair of bell jars was placed over an existing algal patch, another pair was installed at an adjacent site with no algal cover. In the Zostera noltii bed, it was attempted to select sites with maximum and minimum shoot density. Approximately every hour, samples were taken for oxygen determination by a syringe and analysed by Winkler titration. Variation in oxygen content was calculated by a linear regression of the oxygen content over time. In the beginning and the end of incubation lasting 3–5 hrs additional samples were taken by syringes for nutrient analysis (nitrate, nitrite, dissolved phosphate and silicate) following the methods described in Graßhoff et al. (1983). The enclosed macrophytic and macrofaunal biomass (dry weight and ash free dry weight, respectively) was determined at the end of measurements. Parallel to the measurements, the incident light
Because the data sets are comparably small (n
