Planktonic Cnidarians Research Articles

Potential bottom-up and top-down control of large microzooplankton in response to contrasting productive scenarios in the tropical southwestern Atlantic

Large microzooplankton, comprising organisms generally between 64 and 200 μm, plays a significant trophic role in marine ecosystems as primary or secondary consumers. In oligotrophic systems such as the Tropical Southwestern Atlantic, where primary production is dominated by Cyanobacteria, they provide a pivotal link between the basis of food webs and higher trophic levels. In this region, seasonal variations in circulation and continental runoff and wind mixing induce heightened phytoplankton biomass during autumn when compared to a less productive scenario observed in spring, leading to increased abundances of higher trophic levels. In order to establish the connection between primary producers and these higher trophic levels, we investigated the dynamics of large microzooplankton abundance in response to variations in phytoplankton biomass across different systems in the Tropical Southwestern Atlantic. Our findings highlight the complex interactions between bottom-up and top-down control mechanisms that shape large microzooplankton assemblages in these ecosystems. The increase in primary production was accompanied by an observable increase in the abundances of large microzooplankton organisms over the continental shelf, thereby supporting the hypothesis of bottom-up control. In contrast, offshore, in the South Equatorial Current System, a lower abundance of large microzooplankton was observed in the more productive scenario. The intricate relationships between large microzooplankton and higher trophic levels, particularly planktonic cnidarians, appear to be a key driver of these contrasting patterns. The presence of voracious gelatinous predators in the offshore systems, suggests a scenario in which top-down predation may counteract the expected bottom-up response of large microzooplankton to increased phytoplankton biomass. This indicates the importance of considering the entire trophic web when analysing the responses of large microzooplankton to changes in primary production.

Assemblages of planktonic cnidarians in winter and their relationship to environmental conditions in the NW Mediterranean Sea

In the present study, we addressed the spatial characterization and species assemblages of the planktonic cnidarian community (Siphonophorae, Hydromedusae, and Scyphomedusae) in winter, a period that has been the subject of few studies in the NW Mediterranean. Data were obtained on two oceanographic cruises, in February 2017 and 2018. In 2017, the early onset of spring conditions and the subsequent phytoplankton bloom favored a mixture of winter and spring species, resulting in a higher species richness but a lower abundance of cnidarians. However, the typical winter oceanographic conditions in 2018 allowed winter species populations to develop, leading to a higher abundance of cnidarians that year. The most abundant species in both winters were Lensia subtilis, Muggiaea kochii, Chelophyes appendiculata, Abylopsis tetragona (eudoxid), Aglaura hemistoma, and Velella velella rataria larvae, while Obelia spp. was particularly numerous in 2017. In both years, the cluster and redundancy analyses showed a coastal-offshore ordination in species assemblages resulting from the effect of environmental variables (particularly bathymetry) and oceanographic structures (water masses and the shelf-slope density front). The presence of submarine canyons, in which great depths are reached close to the coast, modified the circulation patterns, resulting in a mixture of coastal and offshore species in these areas. In the current scenario of global warming, our results will help to provide a baseline for identifying future changes in the structure of the planktonic cnidarian community.

Distribution and species composition of planktonic cnidarians in the Algerian coastal waters (SW Mediterranean Sea)

This study presents an inventory of planktonic cnidarian species collected in Algerian coastal waters (1°10’00” W - 6°52’37” E and 35°37’42” N - 37°01’16” N) from May 2012 to February 2017. The aim was to analyse the qualitative distribution and identify the main components of this gelatinous fauna in different coastal ecosystems in western, central and eastern regions. A total of 60 taxa of planktonic cnidarians were identified, including 57 hydrozoans and three scyphozoans. The class Hydrozoa were represented by 34 hydromedusae, primarily belonging to orders Anthoathecata (15 taxa) and Siphonophorae (23 taxa). Several reported hydrozoan species were recorded for the first time in the southwestern Mediterranean Sea. These new regional records include the hydromedusae Kantiella enigmatica, Leuckartiara octona, Proboscidactylaornata, Cunina globosa, Lovenella cirrata, Cirrholovenia tetranema, Phialella quadrata, Homoeonema platygonon and the siphonophore Forskalia contorta. Seasonal fluctuations were observed in biodiversity, with the highest species richness in spring and summer. The most common species are the calycophorous siphonophores Abylopsis tetragona, Muggiaea atlantica, Lensia subtilis Muggiaea kochii and the holoplanktonic medusae Aglaura hemistoma, Rhopalonema velatum, Solmundella bitentaculata, Liriope tetraphylla and Pelagia noctiluca. The species’ occurrence frequency exhibited seasonal and spatial variability. These distribution patterns were primarily determined by meroplanktonic hydrozoans (Anthoathecata and Leptothecata).

Amazon River plume habitats shape planktonic cnidarian assemblages in the Western Atlantic.

The impact of the Amazon River freshwater plume on planktonic cnidarians over neritic and oceanic provinces is unknown. To provide further knowledge we took advantage of an oceanographic cruise performed in October 2012 in the Western Atlantic off the North Brazilian coast (8°N, 51°W-3.5°S, 37°W). A complex and dynamic system was observed, with strong currents and eddies dispersing the plume over a large area. Our results show that the Amazon River shapes marine habitats with a thin highly productive surface layer compressed by a deeper oxygen minimum zone both over the shelf and in the open ocean. We hypothesized that such habitat structure is particularly advantageous to planktonic cnidarians, which have low metabolic rates, being able to survive in hypoxic zones, resulting in high species richness and abundance. Over the shelf, distinctions were sharp and the area under the influence of the plume presented a diverse assemblage occurring in large abundance, while outside the plume, the hydromedusa Liriope tetraphylla was dominant and occurred almost alone. Divergences in the oceanic province were less pronounced, but still expressive being mostly related to the abundance of dominant species. We concluded that Amazon River plume is a paramount physical feature that profoundly affects the dynamics of the mesoscale habitat structure in the Western Equatorial Atlantic Ocean and that such habitat structure is responsible for shaping planktonic cnidarian assemblages both in neritic and oceanic provinces.

Temporal dynamics of mesoplanktonic cnidarians in a subtropical estuary: Environmental drivers and possible trophic effects

Planktonic cnidarians are important pelagic predators that occasionally reach extremely high densities. It is important to know how environmental variability affects their abundance and distribution in order to understand the population fluctuations and factors that drive their blooms. Plankton samples collected in the Patos Lagoon Estuary (PLE) during an 8-year period (2009–2017) were analyzed and the mesoplanktonic cnidarian community was quantified. The variability in abundance and composition was associated with environmental variables (temperature, salinity, chlorophyll a, and precipitation). From 234 samples, 23,861 specimens were classified into 34 taxa, of which 21 were determined at the species level. Liriope tetraphylla and Eucheilota maculata were the most frequent species, while Cnidostoma fallax was the most abundant, reaching ~11,369 ind. m−3. The temperature was positively related to richness/diversity, while chlorophyll a did not show a clear influence on the mesoplanktonic community. Under low salinity (<10) the community was characterized by Moerisia inkermanica and C. fallax, while Cunina octonaria and Proboscidactyla ornata were associated with increasing salinity (>25). Liriope tetraphylla and the blooms of C. fallax affected the mesozooplankton community, by drastically reducing copepod density and productivity. Eighteen taxa were considered rare (<10 ind. m−3), which highlighted the importance of higher frequency samplings (≤15 days) in long time series (≥8 years) to detect species with short life span. Our results demonstrate a marked influence of hydrological regime on the composition and abundance of planktonic cnidarians in the PLE.

Sampling planktonic cnidarians with paired nets: Implications of mesh size on community structure and abundance

Mesh selectivity is an important factor to be considered when sampling zooplankton. Selectivity may differ according to the environment and taxon, but his has not been evaluated for cnidarians in neritic and/or oceanic ecosystems. In this study, efficiency of plankton nets with two different mesh sizes (120 and 300 μm) were compared to assess the cnidarian assemblage from neritic and oceanic habitats in the equatorial Atlantic Ocean. No statistical differences between meshes were observed in species richness, total and most dominant taxa abundance, or taxonomic and size structure of the community in both oceanic and neritic habitats. Only for the small (0.5–2 mm) neritic Persa incolorata did the meshes differ, with higher abundances in the finer one. Despite small particularities, our results show that the 120 and 300 μm meshes produce similar results.

Consequences of Stinging Plankton Blooms on Finfish Mariculture in the Mediterranean Sea

In recent years, caged finfish mariculture across European seas suffered production losses by severe fish mortality, following episodic outbreaks of invertebrate cnidarian stingers. Due to their stinging cells and injectable venoms, medusozoan jellyfish or drifting propagules of polyp colonies at high density may impair caged fish health through toxic effects on vulnerable tissues of gills and skin, and related secondary bacterial infections. Gill disorders in European sea bass (Dicentrarchus labrax) fish farms along the Spanish Mediterranean coast are commonly reported, but regular monitoring of the frequency of cnidarian outbreaks and their potential impacts on caged fish is still poorly enforced. In this study, two sea bass mariculture farms in Southern Spain (Malaga; Almeria) were monitored biweekly for zooplankton, phytoplankton and fish gills condition, over 13 or 30 months for the Malaga and Almeria facilities respectively, within the period 2012 – 2014. Significant, direct correlations were found among low water temperature, recorded fish mortalities, and high abundances of planktonic cnidarians, particularly of the hydrozoan siphonophores Muggiaea atlantica and M. kochii, and the larval stage of Ectopleura larynx, a common member of cage biofouling communities. A significant relationship between cnidarian densities and the quantitative scoring of gill pathology was also observed. In addition, high densities of long-bristled planktonic diatoms (Chaetoceros spp.) coincided with a major fish mortality event (April 2012, Almeria farm). Standardised monitoring of plankton dynamics and composition may help in promoting response capacities of Mediterranean mariculture managers to fish health challenges (such as stinging plankton blooms) by a) improving diagnostic tools and preventative countermeasures and b) supporting the development of science-based spatial planning and sustainable growth of coastal mariculture.

Community structure and diversity of gelatinous zooplankton in the Bay of Biscay

Community structure and diversity of gelatinous zooplankton in the Bay of Biscay

Distribution of planktonic cnidarian assemblages in the southern Gulf of Mexico, during autumn

BackgroundDespite their ecological, economic and medical relevance, very little information is available on the distribution of planktonic cnidarians, this being particularly true for some regions of the ocean such as the Gulf of Mexico. In fact, the effect of locally important oceanographic features such as the offshore autumnal transport on the distribution of planktonic cnidarians has long been overlooked in this region. Because of this, the present study aimed to analyse the spatial patterns of planktonic cnidarian assemblages in the southern Gulf of Mexico during the autumn of 1998, when particularly intense conditions of convergence, offshore water transportation and productivity were recorded. The assemblages were described in terms of their composition, abundance (volume), diversity, dominance and equitability. Cluster (Bray-Curtis index) and ordination (multidimensional scaling (MDS)) analysis were performed in order to link the observed distribution of species with the environmental parameters.ResultsSixty-eight taxa were recorded, with Koellikerina fasciculata and Muggiaea atlantica representing new records for the region. The holoplanktonic species Diphyes dispar, Abylopsis spp., Liriope tetraphylla, Diphyes bojani, Aglaura hemistoma, Muggiaea kochi, Chelophyes appendiculata and Eudoxoides mitra were dominant with respect to abundance (ml/1000 m3) and frequency of occurrence and defined the differences among the groups of stations. Four groups of sampling stations were identified: (A) the mouth of the Grijalva-Usumacinta system, (B) the Campeche Bank, (C) the area of offshore oil rigs and (D) the continental shelf off Tabasco and associated oceanic waters. The parameters that defined the groups of stations were zooplankton volume, sampling station depth, salinity and dissolved oxygen concentration.ConclusionsPlanktonic cnidarians are useful as indicators of water masses, since the observed zonation was congruent with the surface circulation patterns that are present during the autumn (offshore transportation) and the presence of particular local environmental conditions, including those at the mouth of the Grijalva-Usumacinta system and those associated with the area of oil rigs. We would expect that similar conditions of strong offshore transport would lead to a similar horizontal stratification in the assemblages of planktonic cnidarians in other parts of the world, driven mainly by salinity, oxygen, depth and zooplankton gradients.

Spatial and temporal variation of planktonic cnidarian density in subtropical waters of the Southern Brazilian Bight

The spatial and temporal variations of planktonic cnidarians were studied across the inner continental shelf (≤40 m) off the State of Paraná in southern Brazil. Monthly zooplankton samples were taken between November 1997 and March 1999 at five stations. The holoplanktonicLiriope tetraphyllaandMuggiaea kochidominated during the entire period, comprising &gt;80% of the specimens. The coast–ocean gradient became evident due to a near-coastal (≤20 m) assemblage formed byL. tetraphylla, Solmaris coronaand meroplanktonic medusae, and a distinct outer (25–40 m) assemblage formed by the medusaeCorymorpha gracilisandAglaura hemistomaand siphonophores. The estuarine runoff during warm rainy periods lowered the salinity (~30) at the inner stations (1, 2 and 3), providing appropriate conditions for the estuarine/coastal species. Wind-driven onshore advection of outer-shelf species to the inner isobaths occurred from late autumn to winter, when the rainfall decreased and salinity increased (&gt;35). Population peaks occurred in both summer and winter at the inner stations. For most species, the seasonal patterns were inconsistent with other studies conducted in the Southern Brazilian Bight, suggesting irregular and unpredictable seasonal distributions of abundance. Whereas in high-latitude ecosystems the dynamics of cnidarian populations follows the seasonal productivity cycle, here, complex hydrographic processes seem to be more important in determining the structure and seasonal dynamics of this community.

Distribution of planktonic cnidarians in response to South Atlantic Central Water intrusion in the South Brazilian Bight

Five oceanographic cruises were made between November 2005 and June 2006, sampling a cross-shelf transect off the South Brazilian Bight (SBB; 26°46′S) to follow the seasonal development of the South Atlantic Central Water (SACW) intrusion over the shelf and its influence on the assemblage of planktonic cnidarians. An onshore wind-driven bottom intrusion of the SACW was clearly perceptible, reaching the coast in January. From March onward, the SACW influence was gradually displaced seaward due to wind and tidal mixing. By late June the SACW influence was offshore and the inshore was dominated by low-salinity waters (<34.5). The abundance, distribution, and general taxonomic composition of both medusae and siphonophores were strongly influenced by the onshore intrusion of the SACW. An inshore–offshore gradient was clear. The Canonical Correspondence Analysis suggested that coastal species – dominated by Liriope tetraphylla, actinula larvae and Muggiaea kochi – were mostly related to food availability and a vertically mixed environment inshore, and their abundance and extent were reduced during intrusion periods. In contrast, species with offshore affinities tended to increase their abundance and distribution during intrusion periods, and were mostly related to the presence of thermal stratification and a deep chlorophyll maximum layer. Most of these offshore species, such as Aglaura hemistoma, Rhopalonema velatum and many calycophorans, are associated with the warm upper layer. However, high concentrations of large (>20mm in diameter) Solmaris corona were observed exclusively in cold waters, suggesting this medusa is a SACW indicator.

Biodiversity and distribution patterns of planktonic cnidarians in San Matías Gulf, Patagonia, Argentina

AbstractThe special location (40–42°S in the Southwestern Atlantic Ocean) and the hydrodynamic regime (limited water exchange with open ocean) in San Matías Gulf (Argentina) seem to produce a particular fauna of planktonic cnidarians whose their abundances are mainly shaped by the Gulf circulation. Four oceanographic cruises, covering 93 stations in three different seasons during 2007 and 2008 were carried out to quantify species richness and abundance, as well as to analyse the distribution of these cnidarians. We identified 20 species of hydromedusae and one siphonophore, increasing the total number of hydromedusae for the area to 23. This value is similar to the one found in the abutting Argentine continental shelf (20), but with a different assemblage composition. Hydromedusae abundances found were low, except for a bloom of the Leptomedusa Obelia spp. during the cold season. The only siphonophore found in the area, Pyrostephos vanhoeffeni, has previously been thought to be endemic to Antarctic and sub‐Antarctic waters, this being the first record for temperate waters of the Southwest Atlantic Ocean.

Cnidaria from the Croker passage (Antarctic Peninsula) with a special focus on Siphonophorae

Studies of “gelatinous” zooplankton are rather rare, and little is known about the biology and ecology of Antarctic Cnidaria, especially for siphonophores. More investigations are necessary for complementing the current information on the “gelatinous” zooplankton inhabiting this important but little know biogeographical region, especially because siphonophores very likely play a significant role in the Antarctic food chains. The species composition, abundance and vertical distribution of planktonic cnidarians in the Croker Passage were evaluated using the data obtained from three expeditions. Zooplankton were sampled with a double plankton net between 0 and 1,200 m water depth in both summer and winter seasons. In total, ten species of siphonophores and four species of medusae were identified. Siphonophore assemblages were dominated by Dimophyes arctica, Pyrostephos vanhoeffeni, and Diphyes dispar. D. arctica and D. dispar occurred in greatest numbers in summer, mainly in the upper mesopelagic zone. P. vanhoeffeni, a cryophilic species, occurred most abundantly in winter (in the lower mesopelagic zone), when they probably reproduce. Restricted circulation from the Bellingshausen Sea over the continental shelf into Croker Passage may exclude the deeper-living oceanic fauna from the area of investigation, as indicated by the low abundances of Crystallophyes amygdalina, Muggiaea sp., and Heteropyramis spp. Four species of medusae were identified. The highest abundance was noted for Solmundella bitentaculata and Arctapodema sp. These occurred most abundantly in the middle (January) and end (April) of summer.

Distribution of net-collected planktonic cnidarians along the northern Mid-Atlantic Ridge and their associations with the main water masses

Abstract Planktonic cnidarians and ctenophores were sampled with a multiple opening–closing net (Multinet) as well as a non-quantitative plankton net along the northern Mid-Atlantic Ridge (MAR) between Iceland and the Azores. Sixty-four species or genera of planktonic cnidarians (38 siphonophora, 21 hydromedusae, 5 scyphomedusae) and one genus of ctenophore were collected. Of these, Leuckartiara adnata and Clausophyes laetmata were new records for the area. Multinet samples collected from depths of 0–100, 100–500, 500–1000, 1000–1500 and 1500–2500 m at 11 stations were compared. Multivariate analysis of the data indicated that species composition and abundance along the ridge varied with the dominant water masses, with changes in the cnidarian zooplankton assemblage observed with regard to geographic location as well as depth. The surface waters of the two northernmost stations characterized by modified North Atlantic Water (MNAW) as well as the three southernmost stations characterized by North Atlantic Central Water (NACW) exhibited relatively high abundances (3284–13,915 individuals·1000 m −3 ) in the upper 100 m. No such peak was evident at the middle stations characterized by Subarctic Intermediate Water (SAIW), where the abundances in the upper three depth strata were consistently lower (57–863 individuals·1000 m −3 ). Across the study area, the lowest abundances were found in the 1500–2500 m stratum (0–56 ind.·1000 m −3 ). The main divergence in the species composition and abundance of planktonic cnidarians was observed at the Subpolar Front (SPF), which marked the boundary for the distribution of many species. The divergence at the SPF was strongest in the upper 500 m but observable down to 1500 m. Profoundly different epipelagic species assemblages were observed in SAIW and NACW on opposite sides of the SPF, with the distribution of several species of calycophoran siphonophores confined to the southern NACW. At mid-water depths, the species composition north of the SPF was possibly influenced by Labrador Sea Water (LSW). The highest diversity of planktonic cnidarians was observed in the surface waters south of the SPF and in the 100–1000 m range north of the SPF.

Diversity and aggregation areas of planktonic cnidarians of the southern channels of Chile (Boca del Guafo to Pulluche Channel)

We identified 36 species of cnidarians (27 hydromedusae, 9 siphonophores) collected from the Moraleda, King, Darwin, and Pulluche channels, located in southern Chile. The hydromedusae Gossea brachymera, Hybocodon unicus, and Sarsia coccometra, and the siphonophores Rosacea cymbiformis and R. plicata were recorded for the first time in the southern channels. Diversity values were higher for Hydromedusae (2.66 bits) than for Siphonophorae (1.15 bits). The largest aggregates were found in the Moraleda and Pulluche channels, with densities that exceeded 7,000 ind·1000 m -3 . The vertical distribution of the dominant species showed Solmundella bitentaculata to be located near the surface (0-20 m), whereas Sphaeronectes gracilis, Muggiaea atlantica, Clytia simplex, and Proboscidactyla ornata were found in subsurface waters (20-50 m) and Amphogona apicata in deeper waters (50-100 m). According to the Pearson analysis, salinity correlated positively with S. gracilis and S. bitentaculata and negatively with P. ornata. C. simplex was indirectly associated with temperature and the dissolved oxygen concentration.

Chemically regulated feeding by a midwater medusa

Planktonic cnidarians are principal predators in the vast habitats between the ocean's surface and the deep-sea floor. Almost nothing is known, however, about the chemical ecology of these fragile midwater animals because of difficulties associated with collecting healthy specimens and con-ducting experiments in the field. With the use of a remotely operated vehicle, we found that the hydromedusa Mitrocoma cellularia is not a passive “drift-net” predator. This relatively simple gelatinous organism reacted to both the taste and smell of prey in the laboratory and in situ. Our results comprise the first definitive demonstration that a species of pelagic cnidarian responds behaviorally to chemical signals, and they lend new insight into the role of chemoreception in structuring mid-water communities.

Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores

We have examined the variability and potential adaptive significance of the wavelengths of light produced by gelatinous zooplankton. Bioluminescence spectra were measured from 100 species of planktonic cnidarians and ctenophores collected between 1 and 3500 m depth. Species averages of maximal wavelengths for all groups ranged from 440 to 506 nm. Ctenophores (41 species) had characteristically longer wavelengths than medusae (34 species), and the wavelengths from siphonophores (25 species) had a bimodal distribution across species. Four species each produced two different wavelengths of light, and in the siphonophore Abylopsistetragona these differences were associated with specific body regions. Light from deep-dwelling species had significantly shorter wavelengths than light from shallow species in both ctenophores (p = 0.010) and medusae (p = 0.009). Although light production in these organisms was limited to the blue-green wavelengths, it appears that within this range, colors are well-adapted to the particular environment which the species inhabit.

Vertical distribution and abundance of mesoplanktonic medusae and siphonophores from the Weddell Sea, Antarctica

The composition, abundance and vertical distribution of mesoplanktonic cnidarians collected along a transect across the Weddell Sea have been analysed. The transect was characterized by a thermocline, approximately between 200 and 100 m, which deepened significantly towards the shelf edges. In total, 10 species of medusae and 18 species of siphonophores were identified. The most abundant medusae were Pantachogon scotti (up to 11,671 specimens/1,000 m3) and Arctapodema ampla (up to 960 specimens/1000 m3). The most abundant siphonophores were Muggiaea bargmannae (up to 1,172 nectophores/1,000 m3) and Dimophyes arctica (up to 230 nectophores/1,000 m3). Five assemblages of planktonic cnidarians were distinguished: (a) epipelagic species located in and above the thermocline; (b) epi- and upper mesopelagic species located in, above and just below the thermocline; (c) epi- and mesopelagic species located in and below the thermocline; (d) mesopelagic species; (e) lower mesopelagic species. Differences in the depth distribution of the various species gave rise to a clear partitioning of the mesoplanktonic cnidarian population throughout the water column. This vertical partitioning was related to the existence of a thermocline, the structure of the water column and the vertical distribution of prey.

Mesoscale coupling between planktonic cnidarian distribution and water masses during a temporal transition between active upwelling and abatement in the northern Benguela system

The composition and spatio-temporal distribution of planktonic cnidarians in the northern Benguela system are described from plankton samples collected on four cruises between December 1981 and March 1982. The sampling period was characterized by initially active coastal upwelling, followed by abatement of upwelling. In all, 20 species of medusae and 24 species of siphonophores were identified. The most abundant medusae were Chrysaora hysoscella, Aequorea aequorea and Liriope tetraphylla, and the most abundant siphonophores were Muggiaea atlantica, Abylopsis tetragona and Bassia bassensis. Three assemblages of planktonic cnidarians were differentiated. The first was associated with coastal upwelling and consisted of the medusae Proboscidactyla menoni, Aglauropsis edwardsii and juvenile C. hysoscella, along with the siphonophore M. atlantica. The second assemblage was associated with oceanic water and the component species were indicative of the offshore influence of the Benguela system; L. tetraphylla and...

Effects of large-scale advective processses on gelatinous zooplankton populations in the northern Benguela ecosystem

In April 1986 a strong intrusion of Angolan waters into the northern part of the Benguela system coincided with an abatement in the upwelling activity that characterizes the region. This important advective process facilitated penetration by large numbers of species and individuals of the gelatinous zooplankton, chiefly siphonophores and hydromedusae. The increased densities in turn were reflected in higher concentration levels and greater diversity in the populations of planktonic cnidarians than are usually generated by the region's upwelling activity. The most conspicuous result was a pronounced latitudmal (north-south) gradient that was clearly detectable above the thermocline, though the effect of the hydrographic event tapered off with depth. The consequences on upwelling regions of interannual events of the scope of that considered here are discussed in relation to the biological characteristics of the siphonophores and rnedusae.