Laguna dos Patos

Lagoa dos Patos is the largest coastal barrier-lagoon on Earth — a 10,000-square-kilometre sheet of shallow water running 155 mi down the Atlantic edge of Rio Grande do Sul in southern Brazil, with a single narrow mouth to the sea at Rio Grande. Behind it sits Lagoa Mirim, shared with Uruguay, and inland the long rivers of the Jacuí and Camaquã that feed the system. This is temperate South America, not the Amazon: winters are cold, the water swings from fresh to brackish with the wind, and the cichlids here are the cool-water southern fauna — chanchita acaras, glittering Gymnogeophagus eartheaters and pike cichlids that overwinter at temperatures most tropical species would never survive. It ranks among the southernmost cichlid waters on Earth.

Geography & hydrology

FEOW places the whole Patos-Mirim drainage in its own freshwater ecoregion, number 334, classed as a temperate coastal river system spanning southern Brazil and eastern Uruguay. The line it draws runs along the coast from around Tramandaí in the north to Piriápolis in Uruguay, with the inland edge set by the divide against the Uruguay River basin; the rivers drain the southern slopes of the Serra Geral and the older granitic Serra do Sudeste before spilling onto the coastal plain. The whole catchment covers roughly 264,0 mi², and it is dominated not by the lakes but by its rivers — the lagoons themselves make up only about 5% of the system's area (FEOW Ecoregion 334; Burns et al., 2015).

The lagoon is a barrier system, not an estuary cut into bedrock. Two long peninsulas of wind-built Holocene and Pleistocene dunes seal off an ancient coastal depression, and the water trapped behind them is what we call Lagoa dos Patos. It is wide and very shallow: three-quarters of its area sits under 7 ft of water, with natural channels reaching 10–16 ft and only the dredged navigation channel going deeper. Lagoa Mirim, smaller at about 3,478 mi², drains north into Patos through the Canal de São Gonçalo, and a low barrage on that canal now blocks salt water from pushing up into Mirim. At the northern tip of Patos the system opens into Lago Guaíba, the broad water on which Porto Alegre sits, and into Guaíba pours the Rio Jacuí — the longest river in the basin — gathering the Taquari, Caí and dos Sinos. The Camaquã and Gravataí empty straight into the lagoon; the Piratini and, into Mirim, the Jaguarão drain the south (FEOW Ecoregion 334; ESSD 14:1015, 2022).

What governs the lagoon is wind as much as river flow. The tide at the Rio Grande inlet is tiny, a mean range near half a metre, so the sea barely reaches in on its own. Instead, the level and the salt are pushed around by the synoptic weather: southerly and southwesterly winds drive marine water in through the 0.5-to-3-km-wide entrance channel, while high river discharge and northerly winds flush it back out. On an annual basis the estuary can run river-dominated, set up a salt wedge, or mix top to bottom, and the salinity front can travel more than 93 mi up the lagoon during strong freshwater pulses. Much of this is tied to ENSO: wet El Niño years drown the system in freshwater, dry La Niña years let the sea climb further in (ESSD 14:1015, 2022; SciRP, 2014).

Water & habitat

The defining gradient here is salt, and it does not sit still. Only the lower lagoon near Rio Grande is reliably brackish; marine and euhaline conditions tend to build through summer and autumn, while oligohaline, nearly fresh water generally takes over in winter and spring when river discharge is high. Because the inlet is so narrow and the tide so weak, it is the balance of wind and runoff that decides where any given fish finds itself on the fresh-to-salt scale in a given month. Salt marshes — the local marismas, stands of Spartina and Scirpus — fringe the lower lagoon and feed the system, and the estuary as a whole is one of the more productive coastal waters on this part of the continent, with nutrient peaks in the tens of micromoles of nitrate and ammonium (FEOW Ecoregion 334; ESSD 14:1015, 2022).

This is subtropical-to-temperate water, and the seasonality is real. Rio Grande do Sul sits far enough south that winters are cool and the rivers follow a mid-latitude rhythm rather than a tropical flood pulse: discharge runs high in late winter and early spring, eases through summer and autumn. Rainfall on the river basins is on the order of 1,8 in a year. Surface water in the open lagoon can warm into the upper twenties Celsius in summer and drop toward 50–59 °F in winter, and the smaller rivers off the Serra do Sudeste swing further still. For the fishes, that cold season is the filter: the cichlids of the far south are built to ride out a genuine winter, and several of the eartheaters here spawn only once water warms again in spring (Burns et al., 2015; ESSD 14:1015, 2022).

Habitat splits along the rivers as much as along the lagoon. Ichthyologists working the tributaries describe two zones: an upper rhithron on the Sul-Riograndense plateau, where the streams are steep, cool, clear and fast over rock and gravel, and a lower potamon on the coastal plain, where the rivers slow into meandering, muddy, well-vegetated channels with fringing floodplains. The Rio Gravataí is the odd one out, running almost its whole length through low coastal wetlands rather than off the highlands. Layered on top are the standing-water habitats the ecoregion is full of — freshwater and brackish coastal lagoons, swamps, marshes and seasonal ponds — including Ramsar-listed wetlands at Lagoa do Peixe on the Brazilian peninsula and the Bañados del Este in Uruguay. That patchwork of fast headwater, slow lowland river, shallow lake and salt-touched lagoon is what spreads the cichlid fauna out across so many niches (FEOW Ecoregion 334; Burns et al., 2015).

The cichlid fauna

About a third of the fishes in this ecoregion are found nowhere else, and the endemism skews toward the small temperate specialists — annual killifishes above all, with fourteen of the sixteen Austrolebias and most of the Cynopoecilus and Rineloricaria confined to these drainages. Against that backdrop the cichlids are not hyper-diverse the way an Amazon tributary's are, but they are the distinctly southern assemblage, and the atlas roster runs across the genera that define cool-water South America: Australoheros, Gymnogeophagus, Crenicichla, Cichlasoma and the wide-ranging Geophagus brasiliensis (FEOW Ecoregion 334; Burns et al., 2015).

The 'chanchitos' are the genus Australoheros — stocky, deep-bodied, barred acaras that aquarists in Europe knew long before the taxonomy was sorted out, because Australoheros facetus was one of the first South American cichlids to naturalise in Iberian and other temperate waters. Five sit on the Patos roster: A. facetus, A. acaroides, A. minuano, A. scitulus and A. taura, the southern set of a genus whose species turn over from one coastal drainage to the next, with A. minuano shared across the middle Uruguay and the Patos-Mirim divide. The eartheaters are the genus that really belongs to the far south: Gymnogeophagus, the only geophagine cichlids adapted to a temperate winter. The roster carries Gymnogeophagus gymnogenys, G. labiatus, G. rhabdotus, G. terrapurpura and G. tiraparae. Several are recent descriptions out of Uruguay and the Mirim drainage — terrapurpura, named in 2016 for its purple-earth sheen and blue-banded flanks, ranges across the lower Uruguay and the Atlantic and Plata coastal drainages, while tiraparae comes from the Rio Negro and the Tacuari in the Mirim basin, a hump-headed eartheater of clear rivers over sand and rock. The genus splits neatly into mouthbrooders and substrate-spawners, and both strategies are present here.

The pike cichlids of Crenicichla fill the predator slot, ambush hunters that lie among rock and root in current. Five are on the roster: the widespread C. lepidota and C. punctata, common right through the rivers and lower lagoon; C. maculata, a larger species of the Uruguay-Plata drainages reaching the southern edge of the system; and C. lucenai, described in 2007 from the upper Rio das Antas in the Jacuí headwaters and known only from this ecoregion. Rounding out the fauna are two generalists that survey after survey pulls from these waters: Cichlasoma portalegrense, the 'Port Alegre cichlid' named for the city at the head of the system, a tolerant lowland acara of slow vegetated water, and Geophagus brasiliensis, the pearl eartheater, which ranges from headwater streams into the brackish lower lagoon and is about as adaptable as a South American cichlid gets. A long-term survey of the Turuçu, one tributary on the plateau-to-plain transect, caught eight of these cichlids alongside lambari tetras, armoured catfishes and marine strays — a fair snapshot of how the family sorts itself across the gradient (Burns et al., 2015; Loureiro et al., 2016; Lucena, 2007).

Conservation

The pressures on this system are the pressures of a populated, farmed, industrial coastal plain rather than a single megadam. The Jacuí-Guaíba drainage at the north end carries the Porto Alegre metropolitan area — more than four million people — and its untreated sewage, industrial effluent and urban runoff, while the lower estuary takes hydrocarbons from the Rio Grande port, refineries and oil terminals; mercury in croaker and catfish flesh runs above background, though below human-consumption limits. The surrounding coastal plain is one of Brazil's great rice bowls, and rice heavily on the rivers and wetlands and returns water carrying agrochemicals and sediment. Engineering has reshaped the system too: the jetties and expanded port at the Rio Grande inlet have changed where sediment settles, and the barrage on the Canal de São Gonçalo, built to keep salt out of Mirim, also shut estuarine and marine species out of the upper lagoon (Frontiers in Marine Science, 2019; FEOW Ecoregion 334).

The fishery tells the clearest cautionary tale. The estuary's marine-and-brackish stocks — black drum, whitemouth croaker (Micropogonias furnieri), the sea catfish Genidens barbus and pink shrimp — were heavily fished from the 1960s on, and the estuarine fishery collapsed in the early 1980s — Genidens barbus, once holding the largest known population of the species, fell to a fraction of its former abundance and never fully recovered. The freshwater artisanal fishery in the upper lagoon has its own problems: forbidden small-mesh gillnets, fished year-round through the November–January closed season, take croaker and mullet below first maturity, while the armoured catfish Loricariichthys anus — caught largely on illegal gear — now makes up most landings, a sign of a community fishing down to whatever is left. Co-management forums and closed seasons exist; enforcement is the recurring weak point (artisanal fishery analysis, J. Fish Biol., 2016; FAO case study; Frontiers in Marine Science, 2019).

Invaders are the newer worry. The golden mussel Limnoperna fortunei, an Asian bivalve that reached the La Plata basin in the early 1990s, is now established through the Delta do Jacuí, Lago Guaíba and Lagoa dos Patos, fouling intakes and rewiring the food web; non-native fishes including common carp, tilapia and the predatory characin Acestrorhynchus pantaneiro have turned up in the catch. The climate is part of the picture too: in April–May 2024 the worst flood on record in Rio Grande do Sul put the Guaíba at Porto Alegre over 17 ft, drowning the city and washing sewage, fuel and debris down into the lagoon — the kind of compound event projected to grow more frequent. The cichlids of Patos are mostly hardy and broadly distributed, and few are individually threatened, but the narrow-range endemics of the headwaters and wetlands, the killifishes especially, sit in the path of rice expansion, urban sprawl and a destabilising climate (golden mussel records, USFWS ERSS; Frontiers in Marine Science, 2019; water-quality study, Sci. Total Environ., 2026).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Australoheros · 7

Cichlasoma · 1

Crenicichla · 4

Geophagus · 1

Gymnogeophagus · 5