Global potential of offshore and shallow waters macroalgal ...
Microecology of macroalgal blooms
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1 J. Phycol. 36, 1–2 (2000) MICROECOLOGY OF MACROALGAL BLOOMS Seaweeds are well known for having diverse and of- ten complex life histories. One life history feature that is common to nearly all seaweeds, however, is the presence of one or more free-living microscopic stages, which exist in a variety of benthic and plank- tonic forms. Despite the important roles played by mi- croscopic stages (reviewed in Santelices 1990, Vadas et al. 1992), much of what we know about factors af- fecting population dynamics and community struc- ture in most seaweeds has come from studies of mac- roscopic forms. Nowhere is this bias in macroscopic ecology more apparent than in the study of the fac- tors that control massive spring blooms of ephemeral algae in temperate and boreal regions of the world. Typically, many of these species are visibly absent for several months of the year during the late fall and winter, yet they recruit in large numbers the following spring. Patterns of relative abundance of ephemeral species have been attributed to differences in their competitive abilities, which generally have been based on the physiological or demographic performance of large adults (Littler and Littler 1980, Wallentinus 1984). Surprisingly few studies have focused on the sources of recruits for these blooms and the degree to which species interactions among microscopic stages (as influenced by their physiological and demo- graphic performance) determine abundance patterns of macroscopic forms. In the western Baltic Sea, eutrophication has caused the flora to shift from dominance by large pe- rennial macrophytes (primarily Fucus vesiculosus and Zostera marina ) to annual filamentous and foliose al- gae. Dense blooms of the filamentous brown alga Pi- layella littoralis appear between March and July of most years. Interestingly, thick mats of green algae of the genus Enteromorpha, which commonly form blooms in coastal waters affected by eutrophication in many other parts of the world, rarely occur in the western Baltic. Using a variety of field and laboratory studies, Lotze et al. (1999) determined that the unusual domi- nance of P. littoralis over Enteromorpha spp. in the west- ern Baltic can be explained by differences in the ecol- ogy and physiology of their early developmental stages, not of adult ecophysiology (Lotze and Schramm 2000). This study showed that the prevalence of P. lit- toralis did not result from a dominance of its micro- scopic forms. Instead, the authors found that the rela- tive abundance of microscopic forms was similar to that observed for macroscopic forms in other eutrophic systems; microscopic stages of Enteromorpha spp. over- wintering on rocks outnumbered those of P. littoralis by as much as 50 to 1. Macroscopic stages of P. littora- lis, however, appeared earlier in the spring and ulti- mately reached a 10-fold higher biomass than Entero- morpha spp., in spite of the latter’s greater abundance of microscopic forms. Results from laboratory experi- ments evaluating the effects of light and temperature indicated that differences in germination rates, rather than the growth of macroscopic forms, accounted for the dominance of P. littoralis; P. littoralis germinated at 58C, whereas germination in Enteromorpha required temperatures of at least 108C. Large numbers of Enter- omorpha germlings appear later in the spring follow- ing the demise of the P. littoralis, but rarely develop into adults. Crustacean and gastropod mesograzers become abundant at this time, and additional labora- tory experiments coupled with subsequent field stud- ies (Worm et al. 1999, Lotze et al. in press) indicate that selective grazing on Enteromorpha germlings and adults prevents it from monopolizing space and re- Fig. 1. Close-up view of an epiphyte bloom of Pilayella lit- toralis dragging down an adult Fucus vesiculosus plant. (Photo courtesy of Heike Lotze.)
Transcript of Microecology of macroalgal blooms
1
J. Phycol.
36,
1–2 (2000)
MICROECOLOGY OF MACROALGAL BLOOMS
Seaweeds are well known for having diverse and of-ten complex life histories. One life history featurethat is common to nearly all seaweeds, however, is thepresence of one or more free-living microscopicstages, which exist in a variety of benthic and plank-tonic forms. Despite the important roles played by mi-croscopic stages (reviewed in Santelices 1990, Vadaset al. 1992), much of what we know about factors af-fecting population dynamics and community struc-ture in most seaweeds has come from studies of mac-roscopic forms. Nowhere is this bias in macroscopicecology more apparent than in the study of the fac-tors that control massive spring blooms of ephemeralalgae in temperate and boreal regions of the world.Typically, many of these species are visibly absent forseveral months of the year during the late fall andwinter, yet they recruit in large numbers the followingspring. Patterns of relative abundance of ephemeralspecies have been attributed to differences in theircompetitive abilities, which generally have been basedon the physiological or demographic performance oflarge adults (Littler and Littler 1980, Wallentinus1984). Surprisingly few studies have focused on thesources of recruits for these blooms and the degree towhich species interactions among microscopic stages(as influenced by their physiological and demo-graphic performance) determine abundance patternsof macroscopic forms.
In the western Baltic Sea, eutrophication hascaused the flora to shift from dominance by large pe-rennial macrophytes (primarily
Fucus vesiculosus
and
Zostera marina
) to annual filamentous and foliose al-gae. Dense blooms of the filamentous brown alga
Pi-layella littoralis
appear between March and July of mostyears. Interestingly, thick mats of green algae of thegenus
Enteromorph
a, which commonly form blooms incoastal waters affected by eutrophication in manyother parts of the world, rarely occur in the westernBaltic. Using a variety of field and laboratory studies,Lotze et al. (1999) determined that the unusual domi-nance of
P. littoralis
over
Enteromorpha
spp. in the west-ern Baltic can be explained by differences in the ecol-ogy and physiology of their early developmental stages,not of adult ecophysiology (Lotze and Schramm2000). This study showed that the prevalence of
P. lit-
toralis
did not result from a dominance of its micro-scopic forms. Instead, the authors found that the rela-tive abundance of microscopic forms was similar tothat observed for macroscopic forms in other eutrophicsystems; microscopic stages of
Enteromorpha
spp. over-wintering on rocks outnumbered those of
P. littoralis
by as much as 50 to 1. Macroscopic stages of
P. littora-lis
, however, appeared earlier in the spring and ulti-mately reached a 10-fold higher biomass than
Entero-morpha
spp., in spite of the latter’s greater abundanceof microscopic forms. Results from laboratory experi-ments evaluating the effects of light and temperatureindicated that differences in germination rates, ratherthan the growth of macroscopic forms, accounted forthe dominance of
P. littoralis; P. littoralis
germinated at5
8
C, whereas germination in
Enteromorpha
requiredtemperatures of at least 10
8
C. Large numbers of
Enter-omorpha
germlings appear later in the spring follow-ing the demise of the
P. littoralis
, but rarely developinto adults. Crustacean and gastropod mesograzersbecome abundant at this time, and additional labora-tory experiments coupled with subsequent field stud-ies (Worm et al. 1999, Lotze et al.
in press
) indicatethat selective grazing on
Enteromorpha
germlings andadults prevents it from monopolizing space and re-
Fig. 1. Close-up view of an epiphyte bloom of Pilayella lit-toralis dragging down an adult Fucus vesiculosus plant. (Photocourtesy of Heike Lotze.)
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ALGAE • HIGHLIGHTS
ducing algal diversity. Thus, the timing of germina-tion appears critical in determining the compositionof spring blooms in the Baltic because conditions forgermling and adult survival become less favorablelater in the year.
Ecologists have made important advances in under-standing the roles played by propagules and micro-scopic stages of algal life histories in the last decade.The rate of discovery will undoubtedly accelerate be-cause new technology is diminishing logistical con-straints that have hampered an understanding ofwhere to find microscopic stages, how to identifythem, and how to manipulate them in nature. This isan exciting age of discovery in seaweed ecology.
Dan ReedMarine Science InstituteUniversity of California
Santa Barbara, CA 93106e-mail [email protected]
Lotze, H. K. & Schramm, W. 2000. Ecophysical traits explain speciesdominance patterns in macroalgal blooms.
J. Phycol.
36:(in press).Lotze, H. K., Schramm, W., Schories, D. & Worm, B. 1999. Control
of macroalgal blooms at early developmental stages:
Pilayellalittoralis
versus
Enteromorpha
spp.
Oecologia
119:46–54.Lotze, H. K., Worm, B. & Sommer, U. 2000. Propagule banks, her-
bivory and nutrient supply control population developmentand dominance patterns in macroalgal blooms.
Oikos
(in press).Littler, M. M. & Littler, D. S. 1980. The evolution of thallus form
and survival strategies in benthic marine macroalgae: field andlaboratory tests of a functional form model.
Am. Nat.
116:25–44.
Santelices, B. 1990. Patterns of reproduction, dispersal and recruit-ment in seaweeds.
Oceanogr. Mar. Biol. Ann. Rev.
28:177–276.Vadas, R. L., Johnson, S. & Norton, T. A. 1992. Recruitment and
mortality of early post-settlement stages of benthic algae.
Br.Phycol. J.
27:331–51.Wallentinus, I. 1984. Comparisons of nutrient uptake rates for Bal-
tic marcroalgae with different thallus morphologies.
Mar. Biol.
80:215–25.Worm, B., Lotze, H. K., Bostrom, C., Engkvist, R., Labanauskas, V.
& Sommer, U. 1999. Marine diversity shift linked to interac-tions among grazers, nutrients and propagule banks.
Mar. Ecol.Prog. Ser.
185:309–14.
