Nutrient limitation

Two criteria were employed to assess nutrient limitation in situ: the response of biomass to nutrient enrichment and the cellular stoichiometry of benthic microalgae.

The increase of total biovolume due to enrichment indicated nutrient limitation in the unenriched treatments. During late spring, summer and autumn, nitrogen was limiting for the benthic microalgae in Kiel Fjord (Table 4.3). For spring 1996, a colimitation of nitrogen and phosphate could be deduced from the fact that microalgal biovolume increased as well with nitrogen as with phosphate enrichment (see also below). Nitrogen limitation seemed to be strongest in the summer and autumn experiments with the highest biomass stimulation (i.e. highest slopes in Table 4.3) compared to the control. In winter and early spring the nutrient treatments failed to produce higher biovolume yields than the control (Table 4.3 and 4.7). It can be assumed that low light conditions (irradiance and daylength) and low temperature in combination with high ambient seawater nutrients (Fig. 2.3) precluded nutrient limitation during winter and early spring. Addition of silicate increased the total biovolume as well, but this increase was significant only in autumn 1997, and in spring 1998 in combination with N+P enrichment. The pattern in spring 1998 indicated a shift towards a Si-limited situation after the addition of N and P. Although the water column concentrations were >15 µmol 1·¹ Si in autumn 1997 (Fig. 2.3), this was obviously not sufficient to meet the demands of the developing biofilm (see below).

The dependency of cellular stoichiometry on nutrient limitation has been established for benthic marine microalgae during this study. On the basis of the optimal ratios and of the limitation indices developed in my laboratory experiments (Chapter 3), the cellular biomass composition in my in situ experiments supported the conclusion that the microflora was nitrogen limited for most parts of the year. C:N was seldom lower than 7.0 and raised up to 13, whereas the N:P ratio <17 and the C:P ratio

<119 indicated a P-surplus. The internal N:P ratios of the biomass ranged from 1 to 64 and increased conspicuously, if media with an increased N:P ratio were supplied (Fig. 4.7). Similar results were obtained by Myklestad (1977) in experiments with two diatom species. From these data, it can be deduced that internal nitrogen pools were not saturated in unenriched treatments ( cf. Droop 1983), supporting the

observed biomass increase in the enriched treatments. The significant correlation indicated also that the addition of nutrients was effective, i.e. the nutrients have been assimilated by the microalgae. This was supported by the decrease of C:N ratios under N-enriched conditions in the second experiment series (Fig. 4.14) Thus, both criteria of nutrient limitation indicated a N-limitation for Kiel Fjord microphytobenthos, at least from late spring to autumn. The response to P-addition in spring 1996 and the N: P variability in spring 1998 indicated a P-limitation at the beginning of the year. This is corroborated by the peaks in N:P ratios in the ambient seawater in spring 1996 and 1998 (Fig. 2.3). The data are not unambiguous, however, because the epilithon had low internal N:P ratios in spring 1996 (Fig. 4.6).

A shift from P-to N-limitation in spring may be caused by high inflow of freshwater, which is more likely to be P-deficient (Hecky et al. 1993). For this I cannot present direct evidence, but the increase in freshwater species in spring may indicate higher inflow from the river Schwentine (Fig. 2.4), which is less than 1 km across the Fjord opposite to my experimental site (Fig. 2.1 ). A shift from P-limitation in spring to N-limitation in summer was reported from several other freshwater influenced ecosystems (D'Elia et al. 1986, Malone et al. 1996). Taking into account also the Si-limitation indicated for diatoms, my experiments showed a mosaic of nutrient effects, which may differ seasonally and for different taxonomic groups. It is therefore a oversimplification to apply Liebig's law of the minimum (limitation by a single nutrient) to natural communities (D'Elia et al. 1986). The changing importance of nutrient conditions during seasonal succession is well understood for freshwater phytoplankton (Sommer et al. 1986). Also for periphyton, nutrients may be important for seasonal development (Chapter 2 and 6), but further research is needed to reveal the combined influence of nutrients, grazing, and abiotic conditions on seasonality of the microphytobenthos.

This study moreover showed that nutrient limitation may still be in effect, even if nutrients are detectable in the water column. The maximum winter concentrations of nitrate in Kiel Fjord clearly exceeded 30 µmol 1·¹ and ambient nutrient pools were

never depleted (Fig. 2.3). Nevertheless, the periphytic algae responded significantly

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to the increased nutrient concentrations. This indicated the existence of diffusion barrieres which limit the access of benthic microalgae to water column nutrients

(Riber & Wetzel 1987, Burkholder et al. 1990). Bothwell (1985, 1989) showed the higher nutrient demands of intact periphyton mats compared to species-specific demands in single-celled cultures. In a review on freshwater periphyton, Wetzel (1996) emphasized the importance of nutrient recycling within the epilithic microflora, since periphyton was shown to be impervious to water currents and thus did not respond to slightly changed water column concentrations of essential resources. He also pointed at an increasing importance of regeneration with increasing biomass of epilithic mats. It can be assumed that regeneration is more effective in natural communities than in my experiments due to the low herbivore abundances on the suspended substrates. By supplying nutrients from the bottom, I presumably overcame this nutrient barrier and additionally reversed the vertical nutrient supply: In natural periphytic mats, the outermost species have access to nutrients first, while in my experiments the nutrients were supplied first to species growing adnate to the substrate. Nevertheless, most of the species responding to nutrient enrichment growed erectly: directly on the substrate (Tabularia), in chains (Me/osira), on stalks (Achnanthes) or in tubes (Berkeleya) (Table 4.4 and 4.8). The vertical position of the mobile unicellular species (Pleurosigma, Proschkinia) could not be determined. A more direct response of erect growing species to nutrient enrichment of the water column was also proposed for freshwater periphyton (Paul

& Duthie 1989, Rosemond et al. 1993). Even in situations where nutrients are

supplied from below the mat, the adnate species may be less effective than erect species at using additional nutrients, since they are limited as well by light. In conclusion, nutrient limitation of epilithic microalgae may be possible in a wide variety of aquatic habitats, not only under oligotrophic conditions. This has implications for the nutrient competition within the epilithon (see below), the trade-off between nutrient availability and herbivore resistance (see Chapter 5) and for the competition between benthic and pelagic microalgae (see Chapter 6).

Epilithic microalgae depend only on nutrients from the water column and recirculation within the biofilm, but sediment-inhabiting microalgae may also use the pore-water nutrients. This may lead to differences in the response of epilithic and epipelic algae to nutrient enrichment (for freshwater, see Blumenshine et al. 1997).

Therefore, a direct application of these results to microphytobenthos on sediments

is not possible. However, studies from marine to freshwater habitats revealed biomass stimulation through nutrient enrichment on a variety of substrates, including sediments (Fairchild et al. 1985, Sundback & Snoeijs 1991, Flothmann & Werner 1992, Rosemond et al. 1993, Coleman & Burkholder 1994, Nilsson 1995). Thus, the conclusion has to be drawn that nutrient limitation is often present in microphytobenthic communities of different substrate quality. This contradicts the assumption by Admiraal (1984) and co-workers that sediment-inhabiting microalgae may not be nutrient limited.