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versión On-line ISSN 0718-560X
Lat. Am. J. Aquat. Res. v.36 n.2 Valparaíso 2008
Lat. Am. J. Aquat. Res., 36(2): 301-307, 2008
On the occurrence of Obelia medusa blooms and empirical evidence of unusual massive accumulations of Obelia and Amphisbetia hydroids on the Argentina shoreline
Bloom de la medusa Obelia y evidencia empírica de una inusual acumulación masiva de hidroides de Obelia y Amphisbetia en la línea de costa en Argentina
Gabriel Genzano*1, Hermes Mianzan2, Luciana Diaz-Briz1 & Carolina Rodríguez1
1 Estación Costera J. J. Nágera, Departamento de Ciencias Marinas, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes 3250 (7600), Mar del Plata, Argentina.
ABSTRACT. Medusa blooms are a scarcely known phenomenon. For temperate, southwestern Atlantic waters, reports of such events are almost nonexistent. This research reports the analysis of a total of 1043 plankton samples taken from 1993 to 2006 and benthic samples collected by seasonal SCUBA diving since 1997 along the Buenos Aires coast. The medusa Obelia longissima was observed in very low densities and frequen-cies, except in October 2003, when a bloom of this species was detected. Soon afterward, two consecutive massive accumulations of O. longissima and Amphisbetia operculata colonies were detected on the coast by the mhabitants of Mar del Plata (38°00'S-57°32'W) and the coastal Mar Chiquita Lagoon (37°32'S-57°19'W) between March and April 2004. The colonies of both species were collected from the shoreline at low tide at both sites. The environmental factors of that period seemed to be particularly appropriate for promoting optimum growth conditions for these hydroid species. This paper reports and describes the mentioned abnormal phe-nomena, providing baseline information on these little-understood events.
Keywords: Cnidaria, Hydrozoa, Leptomedusae, bloom, Buenos Aires coast, southwestern Atlantic Ocean.
RESUMEN. Los blooms de medusas son un fenómeno escasamente conocido. En las aguas templadas del Atlántico sudoccidental, esta clase de eventos son casi inexistentes. Un total de 1.043 muestras de plancton de 1993 a 2006 fueron analizadas como también aquellas de bentos colectadas por SCUBA estacionalmente desde 1997 en la costa de Buenos Aires. Para la medusa Obelia longissima fueron observadas muy bajas densidades y frecuencias, con excepción de octubre de 2003 cuando un bloom de esta medusa fue detectado. Seguidamente, dos acumulaciones masivas consecutivas de colonias de O. longissima y de Amphisbetia operculata fueron detectadas sobre la línea de costa por habitantes de Mar del Plata (38°00'S-57°32'W) y de la laguna costera Mar Chiquita (37°32'S-57°19'W) entre marzo y abril del 2004. Las colonias de ambas especies fueron colectadas desde la línea de costa durante marea baja en ambas localidades. Los factores ambientales en ese período mostraron ser particularmente apropiados promoviendo las condiciones óptimas para el crecimiento de los hidroides de estas especies. Este trabajo reporta y describe el mencionado anormal fenómeno, dando información de base sobre estos eventos poco conocidos.
Palabras clave: Cnidaria, Hydrozoa, Leptomedusae, bloom, costa de Buenos Aires, Atlántico sudoccidental.
The sudden appearance of medusae, with an appar-ently "abnormal" abundance of one or a few species (medusa blooms), is a common but as yet enigmatic characteristic of gelatinous plankton life histories. These events are usually noticed and reported when massive appearances of conspicuous, stinging jellyfish occur near the coastal zone, significantly and visibly affecting human activities such as tourism and fisher-ies (Haddad Jr. et al, 2002).
The question of what a "normal" abundance value is for a particular species remains to be answered. It is quite difficult to properly define a dense concentration of organisms as a bloom. It is possible to accept that medusa blooms occur when the number of individuals of a species increases unusually. In fact, any addi-tional concentration in the number of plankton individuals that surpasses the "average" could probably be considered to be "abnormal", "irregular", "unex-pected", or "exceptional" (Benovic & Lucic, 2001). However, some of the aggregations on beaches may not be the result of a trae bloom, but that of the advection of a population by physical forces (Arai, 1992; Graham et al., 2001). Consequently, of the hundreds of medusa species, only a few of them (e.g. Buecher, 2001; Malej, 2001 and information therein) are known as trae producers of blooms; this identification is only possible when proper baseline information, such as long term monitoring, is available regarding the seasonal cycles and historical abundance values of these species.
If a medusa outbreak occurs some distance from the coast, it would surely pass unnoticed because long-term monitoring in oceanic waters is very uncommon due to the distances and high costs involved (Boero, 1991).
In the temperate southwestern Atlantic waters re-ports concerning these phenomena are almost nonexistent. The few reports of blooms or aggregations, as expected, regarded coastal and stinging species, spe-cifically Olindias sambaquiensis Müller, 1861 (Limnomedusae) and Liriope tetraphylla (Chamisso & Eysenhardt, 1821) (Trachymedusae), locally known as "agua viva" and "tapioca respectively" (Mianzan & Zamponi, 1988; Mianzan et al, 2000).
A bloom of Obelia longissima (Pallas, 1766), a very small Leptomedusae, was detected offshore during the austral spring of 2003 after analyzing a historical series of zooplankton samples colleted since 1993. This bloom was followed by a huge shoreline accumulation of its benthic phase (hydroid colonies) a few months later. Shoreline aggregations of hydroids are very uncommon events that are rarely reported (e.g. Clare et al, 1971) and usually involve a few clumps; these are less noticeable than coastal jellyfish aggregations.
It seems that the environmental factors in austral spring 2003 were particularly appropriate, providing optimum growth conditions for hydroid species, since another larger shoreline aggregation of Amphisbetia operculata (Linnaeus, 1758) colonies occurred in the same area just a month later. This paper reports and describes those abnormal events involving two hydrozoan species off the Buenos Aires coast, Argentina (southwestern Atlantic Ocean).
Ten research cruises were performed along the Buenos Aires coast from the last week of October to the first week of November from 1993 to 2006. A total of 1043 samples were collected with a Pairovet plankton net (200 µ mesh) and analyzed. Obelia medusae were sorted and counted. Their densities were ex-pressed as the number of specimens per cubic meter and their frequency as the percentage of samples with Obelia in the total analyzed samples.
Obelia longissima and A. operculata colonies were collected from the shoreline at low tide from Mar del Plata (38°00'S-57°32'W) to the coastal Mar Chiquita Lagoon(37°32'S-57°19'W).
Populations of both hydroids in the littoral out-crops from Mar del Plata and neighboring areas have been studied seasonally by means of SCUBA diving since 1997; beaches have been checked monthly since 1989 for the presence of clumps. Specimens were preserved in a 5% formaldehyde solution and they were deposited in the authors' collections (Departamento de Ciencias Marinas, FCEyN - UNMdP).
An offshore Obelia medusa bloom and an unusual massive shoreline accumulation of its benthic phase soon afterwards, followed by another hydroid shoreline accumulation of A. operculata, even larger than the previous one, are reported for the first time in the same area.
The Obelia medusa bloom was detected in October 2003. This medusa was found at very low densities and frequencies in the study area between 1993 and 2002. This abundance never surpassed 5 indm-3 . Similar values were reported for Obelia spp. medusae around the world (Vannucci, 1957, 1963; Alvariño, 1968; Vannucci et al, 1970; Goy, 1979; Bouillon, 1995; Pagés & Orejas, 1999; Palma et al, 2007). However, the Obelia frequency was 4 times higher and its density 200 times higher in 2003 than during any previously recorded period. A high proportion of the Obelia were juveniles on these occasions. In the following years (2004-2006), the population decreased, but was still higher than the preceding ones (Table 1, Fig. 1).
Obelia species exhibit a typical leptolid life cycle consisting of a sessile colonial vegetative hydroid stage, a free-living sexual medusa stage, and a planula larval stage. Various nominal species of Obelia hydroids were reported for the Buenos Aires coast (Blanco, 1994; Genzano & Zamponi, 1997, 2003) but there is not way to recognize the medusae species and they are usually named Obelia spp. However; floating bits of Obelia hydroid colonies were also found in some samples along with medusae (Fig. 1). The com-parison of these fragments with reference collections indicated that they corresponded to O. longissima and so it was possible to ascribe the medusae to this species.
Soon after the mentioned medusa bloom, two consecutives massive hydroid shoreline accumulations were detected by the coastal inhabitants of Mar del Plata and neighboring areas; this lasted for more than three months, from the end of February to the end of May 2004. The accumulation was particularly important in Mar Chiquita Lagoon (Fig. 2), a shallow coastal lagoon fed freshwater by a small river and seawater through a narrow southern part during semidiurnal high tides.
Local residents manifested their worries over this unusual event, especially because of the persistent and unpleasant odor of the dead clumps. Local sport fishermen recognized two different kinds of what they called "marine grass": a rare "brown marine grass" at the beginning, followed by a "yellow" one a month later. These colonies were so abundant that they adhered to the fishing lines, hindering sport fishing activities. The clumps also clogged fishing nets (Reta, pers. com.).
The first accumulation corresponded to O. longissima clumps that were found through the end of March 2004. The colonies were up to 17 cm tall, clearly larger and more densely ramified than any other colonies previously collected on the Buenos Aires coast (Genzano, 1994; Genzano et al., 2002; Genzano & Zamponi, 2003) (Fig. 1).
Soon after, in April and May, a second huge accumulation corresponded to Amphisbetia operculata hydroids. The species was so abundant that some clumps were found rolling with the wind on the street next to the lagoon. This species inhabits the rocky sub-littoral outcrops (5-33 m depth). Ephemeral, no feed, and small reduced medusoids are released between May and January. A period of marked colony growth occurs at the end of spring and at the beginning of summer (Genzano et al., 2002).
The presence of pelagic hydroid fragments or en-tire colonies in the water column and their later massive shoreline accumulation are rarely recorded (Clare et al, 1971; Milis, 2001). Clare et al. (1971) reported the occurrence of "rolling colonies" of two hydroid species, Sarsia tubulosa (M. Sars, 1835) and Tubu-laria larynx (Ellis & Solander, 1786), on the shore of Morecabe Bay. Those colonies showed abnormalities (spherical forms) as a consequence of trophosome development in the plankton because chemical pollutants inhibited the larvae from attaching to the substrate. On the contrary, the analyzed O. longissima and A. operculata colonies did not show any signs of abnormalities. Whole trophosomes were detached and then accumulated on the shoreline, probably due to a massive settlement of planulae on an unsuitable sub-strate.
In general terms, under favorable conditions, hydroid colonies grow rapidly; exploit food items that may be highly diluted within the water mass with high assimilation efficiencies (Coma et al., 1995, 1999; Gili et al., 1998; Genzano, 2005); form substantial biomasses; and reproduce. Larval production in me-tagenetic life cycles depends on the colony size and, henee, the number of gonothecae in which the medusae stages are formed. Cornelius (1990) suggested that an average Obelia sp. colony has a potential production of 2000 medusae. Each female medusa could reléase about 20 eggs, for a potential production of 20,000 planulae (assuming that all the medusae survive to release gametes and half are male). Potential planulae production seems to be lower in A. operculata colonies. Only one medusoid is released from each gonotheca, giving a potential production of 400 medusoids and 1,000 larvae per colony, considering that a female contains four to five eggs (G. Genzano, obs. pers.).
Among the factors controlling hydroid reproduction, temperature is a critical one (Gili & Hughes, 1995 for a review). However, for some hydroids, there are many other factors that can induce unusual reproductive activity, such as the response to a deteriorating environment or hormesis (Stebbing, 1982; Cornelius, 1990), and it is now accepted that increased populations may be a consequence of human effects on coastal environments (Purcell et al., 2007; Boero et al, 2008). Consequently, it is necessary to improve the available databases documenting medusa blooms, as this will provide baseline information about these little-studied events.
This work was partially supported by FONCyT, Pict 2006 No. 1553, CONICET PIP 5009, Mar del Plata University, IAC CNR 2076, EXA 372/07. Thanks are due to Dr. Marcelo Pajaro for providing the zooplankton samples (Peces Pelágicos Programm, INIDEP); Dr. Ruth Akselmann (INIDEP) and Dr. Silvia De Marco (UNMdP) for providing abundant colonies; and Mr. Francisco Garcia, Mr. Mario D'Orio, Mr. Mario Palmieri, and Lie. Raúl Reta (UNMdP), who offered valuable information about this event in Mar Chiquita Lagoon. This is INIDEP Contribution No. 1521.
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Received: 31 March 2008; Accepted: 25 Jun 2008