Control of macroalgal blooms at early developmental stages: Pilayella littoralis versus Enteromorpha spp.

Abstract

Although blooms of opportunistic fast-growing macroalgae now occur frequently in coastal ecosystems affected by eutrophication, their initiation and control is little understood. Most previous studies have focused on the ecophysiology of adult algae only. We show that spores and/or germlings may represent critical stages in the life cycles and mass-developments of co-occurring opportunistic macroalgae in the Baltic (Pilayella littoralis and Enteromorpha spp.). We investigated the overwintering of spores, timing of germination, subsequent growth, and grazing on spores and germlings, in order to explain the initiation of mass blooms and species dominance patterns. In the field, Enteromorpha spp. showed 10- to 50-fold higher abundances of overwintering microscopic forms (up to 330 individuals cm) than P. littoralis. Moreover, we found continuous production of spores (up to 1.2 million settling spores m h) from April to October in Enteromorpha spp., while there was evidence of only a short reproductive period in Pilayella. However, in spring, germlings and adults of P. littoralis appeared earlier in the field and reached a 10-fold higher biomass than Enteromorpha spp. In factorial laboratory experiments including temperature and light, there were clear differences in timing of germination. P. littoralis germinated at 5°C whereas Enteromorpha spp. required temperatures of 10-15°C for germination. In contrast, we detected only minor differences in growth response among adults of P. littoralis and Enteromorpha spp. Germination, not growth of adults, appeared to be the ecophysiological bottleneck for initiating mass spring development. Following the spring Pilayella bloom, Enteromorpha germlings occurred massively in the field (April-September), but rarely developed into adults. In laboratory feeding experiments we tested whether crustacean mesograzers common in summer may control development of Enteromorpha germlings. Both germination of settled spores and growth of germlings were reduced by 93-99% in the presence of grazers (Idotea chelipes and Gammarus locusta). Thus in addition to ecophysiological constraints, grazers, if present, may play a decisive role in the early life stages of macroalgal mass developments. These results mirror patterns of overwintering of seeds, germination control, seed and seedling predation in terrestrial plant communities.

Effect of nutrient inputs on growth, chlorophyll, and tissue nutrient concentration of Ulva reticulata from a tropical habitat

“…There was little evidence for additive effects of N+P. On the contrary, uptake of each nutrient (NO 3 -, NH 4 + and HPO 4 2-) was slightly higher when it was added alone than in combination with others (p < 0.001).…”

“…Two-way ANOVA was used to test differences among uptake rates of N as NO 3 -or NH 4 + alone or in combination with P. Significant differences between pairs of means were identified using the Tukey test 28 .…”

“…The maximum uptake rate was found in the case of the NH 4 + treatment (V max = 9.39 ± 1.26 μmol g -1 DW h -1 1-2 h of incubation). After 24 h, approximately 96% of the added NO 3 -and approximately 90% of the added NH 4 + was taken up. At the end of the experiment N was depleted in both NO 3 -and NH 4 + treatments (Figs.…”

“…After 24 h, approximately 96% of the added NO 3 -and approximately 90% of the added NH 4 + was taken up. At the end of the experiment N was depleted in both NO 3 -and NH 4 + treatments (Figs. 1A, 1B).…”

Seasonal variability in the grazing potential of the invasive amphipod Gammarus tigrinus and the native amphipod Gammarus salinus (Amphipoda: Crustacea) in the northern Baltic Sea

“…Although G. salinus is able to remove only 40% of algal production, it is important to stress here that E. intestinalis has low biomasses and small spatial extent in spring as the species is within its early stage of the annual succession (Kiirikki and Lehvo 1997;Kotta et al 2006). Therefore G. salinus may actually control the development of E. intestinalis in spring, especially within smaller patches where gammarids have a potential to deplete the algal species (Lotze et al 1999). Removal of the efficient grazers may disrupt this natural equilibrium and initiate extensive blooms of E. intestinalis.…”