Ricardo Scrosati

The invasive kelp Undaria pinnatifida (Phaeophyceae, Laminariales) reduces native seaweed diversity in Nuevo Gulf (Patagonia, Argentina)

The kelp Undaria pinnatifida(Phaeophyceae) is a seaweed native to northeast Asia, but during the last two decades, it has been accidentally or intentionally introduced in several temperate coasts worldwide. In central Patagonia (Argentina), this species was first detected in late 1992, and it is progressively spreading from the point of introduction. Through a manipulative experiment involving Undaria removal in 2001, we found that its presence is associated with a dramatic decrease in species richness and diversity of native seaweeds in Nuevo Gulf. Future prospects are worrisome, as, in addition to the negative impact from a biodiversity viewpoint, native commercial macroalgae and invertebrates might also be affected.

THE IMPACT OF FROND CROWDING ON FROND BLEACHING IN THE CLONAL INTERTIDAL ALGA MAZZAELLA CORNUCOPIAE (RHODOPHYTA, GIGARTINACEAE) FROM BRITISH COLUMBIA, CANADA

The importance that frond crowding represents for the survival of fronds of the clonal intertidal alga Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (Rhodophyta, Gigartinaceae) was investigated in Barkley Sound, British Columbia, Canada. Frond density is high for this species, up to 20 fronds·cm−2 in the most crowded stands. Frond crowding imposes a cost in the form of reduced net photosynthetic rates when fronds are fully hydrated as a result of reduced irradiance compared with experimental (not found naturally) low‐density stands. However, the interaction between desiccation and irradiance alters this relationship between net photosynthetic rates and frond density. During a typical daytime low tide in spring, irradiance is 10–30 μmol·m−2·s−1 below the canopy of fronds, and frond desiccation (relative to total water content) can reach 43% at the end of the low tide. In contrast to natural stands, fronds from experimentally thinned stands are subjected to irradiances up to 2000 μmol·m−2·s−1 because of the spatial separation among fronds and can desiccate up to 81% at the end of the same low tide. Laboratory experiments showed that negative net photosynthetic rates occur between 40% and 80% desiccation at an irradiance of 515 μmol·m−2·s−1, and the literature suggests that strong bleaching could occur as a result. At 20 μmol·m−2·s−1 of irradiance and desiccation levels up to 40%, simulating understory conditions of natural stands, net photosynthetic rates are never negative. Experimental thinning of stands of M. cornucopiae done during spring effectively resulted in a stronger extent of frond bleaching compared with natural stands. Therefore, the cost of reduced net photosynthetic rates at high frond densities when fronds are fully hydrated is counterbalanced by the protective effects of frond crowding against extensive bleaching, essential for survival at the intertidal zone. Future research will have to demonstrate the possible relationship between the frequency and duration of negative net photosynthetic rates and the extent of frond bleaching.

An updated definition of genet applicable to clonal seaweeds, bryophytes, and vascular plants

Summary The genet of clonal plants is currently defined as the genetic individual that develops from the zygote and that produces ramets vegetatively. Genetic individuality refers to the fact that it is considered to be a genetically uniform plant. However, somatic mutations may give rise to genetic mosaics within the genet, invalidating the assumption of its genetic uniformity. On the other hand, zygotes are not the only initial cells of genets, since parthenogenetic female gametes may also develop into new individuals directly. Sexual and asexual types of reproduction are similar in terms of dispersal, dormancy, recruitment, and seedling development, especially compared with vegetative fragmentation, process that does not create new genets. For other clonal autotrophic macroorganisms, such as clonal bryophytes and red seaweeds, new individuals that result from reproduction develop from a variety of spores, not from zygotes. These considerations suggest that the existing definition of genet should be updated. For clonal autotrophic macroorganisms, in general, the genet may be defined as the free-living individual that develops from one original zygote, parthenogenetic gamete, or spore and that produces ramets vegetatively during growth. The degree of genetic differentiation between consecutive generations of genets will primarily depend on the type of reproduction involved. Der Genet klonaler Pflanzen ist definiert als das genetische Individuum das sich aus einer Zygote entwickelt und das auf vegetative Weise Rameten produziert. Der Begriff der genetischen Individualitat besagt, dass das Individuum genetisch homogen ist. Jedoch konnen somatische Mutationen innerhalb des Genets dazu fuhren, dass sich ein genetisches Mosaik entwickelt, das die genetische Individualitat des Organismus verletzt. Andererseits konnen Geneten sich nicht nur aus Zygoten entwickeln, aber auch parthenogenetisch von weiblichen Gameten abstammen. Sexuelle and asexuelle Fortpflanzung gleichen einander in Verbreitung, Ruhezustand, Rekrutierung, und Keimlingsentwicklung, speziell verglichen mit vegetativer Fragmentation, einem Prozess bei dem sich keine neuen Geneten entwickeln. Neue Individuen von anderen klonalen, autotrophen Makroorganismen wie klonale Bryophyten und Rotalgen entwickeln sich aus einer Vielfalt von Sporen, nicht aus Zygoten. In dieser Hinsicht sollte die Definition von Genet fur klonale autotrophe Makroorganismen aktualisiert werden. Generell kann der Genet als das freilebende Individuum definiert werden, das sich aus einer Zygote, einem parthenogenetischen Gameten oder einer Spore entwickelt, und das Gameten auf vegetative Weise produziert. Der Grad der genetischen Differenzierung zwischen konsekutiven Generationen hangt primar von der Reproduktionsart ab.

Persistence of gametophyte predominance in Chondrus crispus (Rhodophyta, Gigartinaceae) from Nova Scotia after 12 years

Gametophytes predominated clearly over tetrasporophytes in an intertidal population of Chondrus crispus at Tor Bay (Nova Scotia, Canada) in the summer of 1991. Since this species is perennial and the rocky substrate is stable at this site, we predicted that gametophyte predominance would persist after several years. We confirmed this hypothesis by re-sampling the same area in the summer of 2003. This is one of the first long-term studies of the relative abundance of life-history phases done unequivocally at the same site for the Gigartinaceae.

Feeding preference of Littorina snails (Gastropoda) for bleached and photosynthetic tissues of the seaweed Mazzaella parksii (Rhodophyta)

Marine invertebrate herbivores occasionally prefer particular tissues of a given seaweed depending on tissue age. For the intertidal alga Mazzaella parksii (=M. cornucopiae), however, important tissue differences result from abiotic stress: distal tissues of many fronds become bleached under strong irradiance and desiccation during spring and summer low tides. Snails of the genus Littorina (periwinkles) are significant grazers of M. parksii. Through a multiple-choice feeding-preference experiment, we found that periwinkles actively consume bleached tissues, but almost no photosynthetic tissues. This suggests that bleached tissues of M. parksii may support Littorina populations to a good extent during spring and summer. In addition, since photosynthetic tissues are basal, the impact of periwinkles on frond mortality might be lower than if they preferred photosynthetic tissues. Photosynthetic tissues are actively consumed by periwinkles when they are the only food choice, as shown by other researchers. Therefore, retaining bleached tissues for some time might have evolved in M. parksii to divert grazing pressure away from vital tissues.

Interannual variation of the abundance of Mazzaella cornucopiae (Rhodophyta, Gigartinales) from Pacific Canada in relation to changes in abiotic variables

The seaweed Mazzaella cornucopiae (Postels & Ruprecht)Hommersand is common in rocky intertidal areas from Pacific Canada andis a potential economic resource. In both 1993 and 1994, the abundanceof M. cornucopiae from Prasiola Point, southern Barkley Sound, washigh in spring and summer and low in fall and winter. In 1995, however,the abundance in summer was unexpectedly low, and this trend deepenedin 1996. Correlations between the temporal changes of abundance and ofsome abiotic variables were done as a first approach to explaining thesechanges of abundance. The abiotic variables used were air temperature,sea surface temperature, wave height, all three measured on an oceanicbuoy close to Prasiola Point, and seawater salinity, determined for coastalwaters from northern Barkley Sound. These were the closest sites toPrasiola Point for which reliable abiotic data existed. None of thecorrelations were significant. Field observations done at Prasiola Pointsuggest that air temperatures reached higher values there than at theoceanic buoy. Together with irradiance, in situ air temperature may havehad an important role in the interannual differences of abundance througha higher physical stress on thalli, resulting in the high proportion ofbleached tissues observed in summer 1996. Future studies on thepopulation dynamics of M. cornucopiae should benefit fromquantifying these variables in situ.

ON THE ANALBIS OF SELF‐THINNING AMONG SEAWEEDS1

Plant self-thinning is the process that involves density-dependent mortality of plants that are actively growing in crowded conditions in a monospecific, even-aged stand (Weller 1987). This process has mainly been studied in terrestrial plants, but evidence that it occurs among seaweeds is also available (Black 1974, Dean et al. 1989, Reed 1990, Ang and De Wreede 1992, Creed 1995). A recent note (Flores-Moya et al. 1996) presented an analysis of self-thinning as it occurs for two seaweeds: the kelp Phylluriopsis puqurascens ( C . Agardh) Henry et South and the red species Asparagopsis armata Harvey. The analytical approach that was followed in that study was unfortunately inadequate in part, but it could be improved upon. The problems, which will be outlined below, have already been discussed in the ecological literature, yet awareness among ecologists, including those acting as either authors or manuscript reviewers, does not seem to be widespread. The objective of this comment is to identify and discuss these problems and to suggest a better analysis of self-thinning among seaweeds that could help improve future efforts. The first problem relates to the appropriate choice of variables that are used to detect and describe self-thinning through correlation and regression analyses. Originally, Yoda et al. (1963) examined the temporal variation of the relationship between the logarithm of mean plant biomass and the logarithm of plant density for crowded stands during active growth, and subsequent authors followed that choice (see a list of relevant papers in Weller 1987). However, Weller (1987) later showed that this approach presents problems for the interpretation of data, suggesting that total stand biomass (expressed per unit area) be used instead of mean plant biomass. For example, if 1) small plants die and the growth of large plants just barely compensates for that loss of biomass (i.e. at the end of the growth season, when growth rates slow down), stand biomass will not vary while plant density decreases. If 2) later the growth of large plants stops and their biomass remains constant for a while, but some small plants are still dying, stand biomass will be negatively related to plant density. conditions 1 and 2 do not represent active growth of plants. Active growth occurs during self-thinning and involves the highest

Regeneration and reproduction of Mazzaella cornucopiae (Rhodophyta, Gigartinaceae) after frond harvesting

The effects that different intensities of frond harvesting have on frond regeneration and subsequent production of reproductive structures were investigated for the red intertidal alga Mazzaella cornucopiae (Postels & Ruprecht) Hommersand from British Columbia, Canada. Harvesting was done by pruning fronds in the late spring (when stand biomass is highest) of 1993 at two intensities: total and partial collection of fronds, in this second case leaving all frond biomass less than 1 cm high in place. Holdfasts were not damaged. Total percent cover of thalli, frond density, mean frond length, and stand biomass for these experimental quadrats were statistically similar to values for control quadrats in the spring of 1994. These results suggest that one total harvest of fronds per year, done in late spring without damaging holdfasts, may give the highest sustainable yield of biomass. The effects of harvesting intensity on reproduction were variable and difficult to explain. Neither the appearance nor the abundance of cystocarpic fronds were affected by frond pruning, compared with control areas, but pruning did affect the appearance and the abundance of tetrasporic fronds. Partial pruning resulted in a longer presence of tetrasporic fronds, whereas total pruning was associated with their complete absence. Results are compared with those for the few other species of the Gigartinaceae for which experimental harvesting has been done.

The relationship between stand biomass and frond density in the clonal alga Mazzaella cornucopiae (Rhodophyta, Gigartinaceae)

The negative relationship between stand biomass and plant density observed in terrestrial plants was tested among fronds of a clonal red alga, Mazzaella cornucopiae. Stand biomass and frond density were estimated bimonthly for 1 year on 7 permanent quadrats. A positive linear correlation was found between biomass and density for the whole data set, suggesting the lack of self-thinning among fronds of this intertidal alga at natural densities. Higher frond densities could favor increased water retention among fronds, thus minimizing desiccation during low tides. In this way, stands could maintain higher production of biomass. Fronds may also be cushioned better against wave action at higher frond densities, thus decreasing the detachment of larger fronds. The temporal variation of the relationship between biomass and density was plotted separately for these 7 quadrats. Four quadrats showed positive linear correlation between both variables (the other three quadrats showed non-significant positive linear correlation). Their four slopes are statistically similar to that found for the entire data set. It is possible, then, that there is only one positive slope for the biomass-density relationship in this population. If this is true, standing biomass could be estimated from the density of fronds.

A discrete-time logistic model of frond dynamics for Mazzaella parksii (Rhodophyta, Gigartinales)

Mathematical modelling is useful in population ecology and resource management. Logistic models have traditionally been applied to unitary organisms, but it is unclear whether they could be used at the frond (ramet) level for clonal seaweeds. This study shows that frond dynamics for the clonal seaweed Mazzaella parksii (=M. cornucopiae) can be described by a discrete-time logistic model. The model is realistic in that it includes density-dependence, which was previously demonstrated experimentally for this species, and only necessitates data on frond density measured at discrete time intervals. This may constitute a useful tool for the management of clonal seaweeds of economic importance that occur in dense stands.