Keywords

• Chlorophyll a;
• Diatoms;
• Growth rate;
• Nitrate;
• Phosphorus;
• Silicate

1. Introduction

Increasing nutrient enrichment in coastal waters, generally accompanied by changing ratios of nutrients due to their disproportionate inputs, has been shown to profoundly affect phytoplankton species composition and production (Aktan et al., 2005). An increase in marine phytoplankton biomass usually begins following the delivery of exogenous nutrients by processes such as terrestrial outflow, upwelling, nitrogen fixation and atmospheric deposition (Mackey et al., 2009). Sufficient nutrients and light and the absence of grazers are conditions that stimulate phytoplankton growth leading to blooms. However, the effects of nutrient enrichment are highly species-specific (Piehler et al., 2004), and the response to episodic perturbations depends on the initial composition of the phytoplankton community (Estrada et al., 1987).

The ratio of nutrients supporting phytoplankton growth has long been of interest in aquatic science, primarily as a factor affecting succession (Flynn, 2010). The elemental composition of phytoplankton is known to vary considerably depending on ambient nutrient concentrations (Hecky et al., 1993), light and temperature (Goldman, 1986) and growth strategies (Arrigo, 2005). The Redfield ratio (Redfield, 1958), which describes the relationship between organism composition and water chemistry, is often used as a benchmark for differentiating nitrogen (N) from phosphorus (P) limitation by applying the criterion that phytoplankton is N-limited at N:P < 16 and P-limited at N:P > 16 (atomic ratios). However, the value of the N:P ratio for particulates tends to be much less than 16 (median 9). It is probable that this outcome results from the accumulation of inorganic P storage products (Geider and La Roche, 2002). Data from La Roche et al. (1993) suggest that different phytoplankton taxa are characterised by different forms of C:N:P stoichiometry under nutrient-rich conditions.

In a recent review on nutrient enrichment in marine, freshwater and terrestrial ecosystems, Elser et al. (2007) concluded that combined enrichments produce similarly strong synergistic effects in all habitats and that N and P limitation appear to be equally important in terrestrial and freshwater ecosystems, whereas N limitation is stronger in marine systems.

Sandy beaches with well developed surf-zones are known to host dense populations of diatoms, which may accumulate in patches of discoloured water and require a large supply of nutrients for growth (Lewin et al., 1989 and Campbell, 1996). At the sandy and exposed Cassino Beach in southern Brazil, the diatom Asterionellopsis glacialis (Castracane) Round is the main microalga in the surf-zone and frequently dominates the phytoplankton. This diatom is the primary food source for the beach zone's rich fauna of mollusks, crustaceans and fishes ( Garcia and Gianuca, 1997). High concentrations of A. glacialis in the surf-zone of Cassino Beach usually follow the passage of polar fronts with onshore blowing southerly winds, which generate high wave energy, the resuspension of sediments containing benthic seeds in the backshore and the transport of these seeds towards the beach ( Odebrecht et al., 1995 and Rörig and Garcia, 2003).

At Cassino Beach, after the deposition of substantial amounts of mud on the beach in 1998 and the subsequent occurrence of other large mud deposition events, the bloom frequency of A. glacialis decreased together with other species (Skeletonema sp., Campylosira cymbelliformis (A. Schmidt) Grunow ex Van Heurck, 1885, Thalassiosira spp.) normally present in high abundance ( Odebrecht et al., 2010). Cassino Beach is near the mouth of Patos Lagoon, which transports sediments to the coastal zone. Mud deposition episodes are recurrent phenomena associated with the discharge of large amounts of suspended matter from the Patos Lagoon and the subsequent formation of muddy sediments along ~ 50 km of the shoreface, although the 1998 event was of extreme magnitude ( Calliari et al., 2007). The reduction of the surf-zone diatoms after major mud deposition events may have been caused by physical and chemical changes in the environment due to the influence of the mud and deserves further study.

This study will explore the influence of inorganic nutrients on the surf zone phytoplankton of Cassino Beach. The aim of the study was to analyse the short-term (4–5 days) responses of the chlorophyll a content, cell density and species composition of the phytoplankton from the surf-zone of Cassino Beach in response to different concentrations of dissolved inorganic nutrients.

2. Materials and methods

Two laboratory experiments were conducted with subsurface surf-zone water from Cassino Beach, RS, Brazil (32°16′06.9″ S; 52°14′30.8″ W) during the summer (S, February 2) and the winter (W, August 23) of 2010. Measurements of surface water temperature and salinity were made in situ (temperature S 26.7 °C, W 14.2 °C and salinity S 28, W 26). In the laboratory, the water samples were placed in glass bottles (1 L Erlenmeyer flasks) after the water was passed through 200 μm nylon mesh to remove large zooplankton. All incubations were performed with three replicates per treatment in a chamber with controlled conditions of temperature (S 25 ± 3 °C, W 18 ± 1 °C), salinity (S 28, W 26), light (100 μmol m^− 2 s^− 1) and a 12:12 light–dark period.

Inorganic nitrate (NaNO₃), phosphate (NaH₂PO₄) and silicate (Na₂SiO₃.9H₂O) were added (Table 1) to the incubation flasks using autopipettes. Controls (NoNut) were also prepared. Each treatment was sampled daily for five days in the summer experiment and four days in the winter experiment. Inorganic dissolved nitrate, phosphate and silicate concentrations were determined at the beginning of each experiment using standard spectrophotometric methods (Strickland and Parsons, 1972). During the summer, the natural environment contained the following concentrations of the specified nutrients: SiO₃^2 − 19.98 μM, NO₃⁻ 2.38 μM and PO₄^3 − 0.80 μM. The corresponding concentrations during the winter were SiO₃^2 − 14.73 μM, NO₃⁻ 4.31 μM and PO₄^3 − 1.48 μM (Table 1).