Guianas

Geography & hydrology

The Guianas freshwater ecoregion, as drawn by Freshwater Ecoregions of the World, takes in every independent river running off the northern and eastern slope of the Guiana Shield into the Atlantic, from the Demerara in Guyana east to the Oyapock on the French Guiana–Brazil border (FEOW, Ecoregion 311). Its southern limit is the Amazon drainage divide and, to the west, the Essequibo and Orinoco systems; what falls between is a belt of medium and small coastal rivers — Demerara, Corantijn (Corentyne), Coppename, Saramacca, Suriname, Maroni/Marowijne, Sinnamary, Approuague, Oyapock and their neighbours — that share a geology and a fauna rather than a single trunk channel.

The defining substrate is the Guiana Shield itself, a fragment of Precambrian craton and one of the planet's oldest and most stable land surfaces. Most of the ecoregion sits on flat coastal lowlands; the relief is in the interior highlands — the Wilhelmina, Bakhuis, Kayser and Oranje ranges of Suriname, rising to Juliantop at 1,938 ft (about 4,220 ft) in the Wilhelmina Mountains (FEOW). The result is a two-part river profile. In the highlands and on the shield's edge, rivers run as moderate-gradient streams broken by rapids, cataracts and waterfalls — Guyana's Kaieteur Falls is the spectacular case. Where they reach the coastal plain they slow into low-gradient channels, blackwater creeks, savanna pools, lakes and seasonally flooded swamp forest, with a near-continuous belt of inundated swamp running from the Corantijn to the Marowijne (FEOW). There is no Andean meltwater and no great inland floodplain here; the seasonal rhythm is set by the tropical wet and dry seasons and the modest rise and fall they drive across flat terrain.

A second geographic theme runs through the fish biology: an east–west grain. Because each river reaches the sea independently, faunas have been partly isolated drainage by drainage, and the alternation of glacial and interglacial sea levels — which repeatedly joined and severed neighbouring river mouths across the exposed continental shelf — has left a marked east-to-west gradient in species distributions across the Guianas (de Mérona et al., Cybium, 2012). It is a landscape that makes endemics.

Water & habitat

Shield water is, as a rule, soft, acidic and poorly conductive — the standing description of French Guianan rivers in the regional literature (de Mérona et al., 2012). With almost no soluble rock to weather, the rivers carry very low dissolved-ion loads and low electrical conductivity, and their pH sits below neutral, often well into the acid range in forest creeks stained dark by humic and fulvic acids leached from sandy soils and deep leaf litter. This is the blackwater end of the spectrum: optically the water can be clear of suspended sediment yet steeped tea-to-coffee dark, so a stream can be transparent and acidic at once. Other reaches over sand and rock run genuinely clearwater. What the region lacks, in its natural state, is the muddy, mineral-rich whitewater of the Andes — until gold mining adds it artificially.

The habitats that matter for cichlids are the small ones. Above the coastal plain, fish live among forest creeks (the local criques), shaded pools, rock and root tangles, submerged wood and leaf packs, and the margins of rapids — though notably no Guianan fish is strictly confined to the fast rapid habitat itself; creeks and pools, not riffles, hold the distinct communities (de Mérona et al., 2012). Habitat structure does the sorting: bank cover, overhanging canopy, aquatic vegetation, conductivity and dissolved oxygen are the variables that best predict which species occupy a given stretch. On the coastal plain, the seasonally flooded swamps, savanna creeks and tannin-dark backwaters are the classic dwarf-cichlid country — slow, soft, acidic, wood-rich water with abundant cover. The keyhole cichlid (Cleithracara maronii) is the textbook example: it inhabits small coastal creeks of clear, slow water that are rich in decaying wood, where a startled fish will press itself flat against a sunken log and shift its colour to match (FishBase, after Kullander & Nijssen, 1989).

The waters are warm and equatorial year-round, on the order of the mid-20s °C (roughly 75–82 °F) in shaded forest streams, and the temperature swing is small. As elsewhere on the shield, the meaningful gradients are spatial and structural — channel width, depth, transparency, conductivity and oxygen separating one reach from the next — rather than the deep seasonal or thermal cycles of a temperate river or a rift lake.

The cichlid fauna

The Guianas hold a compact but distinctive cichlid assemblage, and a striking share of it is endemic to the shield. The flagship is Guianacara, the bandit acaras — medium-sized, boldly bar-masked cichlids that Kullander and Nijssen erected in 1989 and that form essentially the only cichlid clade endemic to the Guiana Shield, with several species across these coastal rivers (G. geayi, G. owroewefi, G. sphenozona, G. oelemariensis) sifting moderately flowing, structured water over sand and rock. Alongside them sit the region's two characteristic 'acara' lineages: Krobia, the dolphin cichlids (K. itanyi, K. guianensis), medium generalists of creeks and flooded margins, and the dwarf Nannacara — the golden dwarf cichlid (N. anomala) and the goldenhead (N. aureocephalus) — small, sexually dimorphic substrate-spawners that thrive in soft, acid, leaf-littered water. Add the truly miniature, shield-endemic Mazarunia, dwarf geophagine cichlids no longer than about 3.5 in restricted to the upper Mazaruni River in Guyana, and the picture is of a genuine dwarf-and-medium radiation built on this old, ion-poor drainage.

The most famous aquarium export is the keyhole cichlid, Cleithracara maronii — its single genus named for the Maroni River, its range running along the coast from the Barima in Guyana to the Ouanary in French Guiana, with a foothold in the Orinoco delta (FishBase). It is a peaceful, biparental, pair-spawning cichlid of wood-choked creeks, and the 'keyhole' is the dark blotch on its flank.

The rest of the fauna reads as a Guiana-Shield edition of the broader Neotropical cichlid playbook. Eartheaters are well represented — Geophagus (the Maroni eartheater G. harreri, the Oyapock G. camopiensis, plus G. surinamensis and G. brachybranchus) and the sand-sifting Satanoperca and Retroculus of the rapids. Pike cichlids of the genus Crenicichla are diverse, from the widespread ringtail (C. saxatilis) to local forms like C. nickeriensis and the data-deficient C. sipaliwini. The apex predators are the peacock basses, Cichla ocellaris and C. monoculus, sight-hunters that do best in the clearer, warmer water. Smaller niches are filled by the dwarf Apistogramma (A. steindachneri, A. gossei, the Rupununi-savanna A. wapisana), the flag cichlid Mesonauta guyanae, the angelfish Pterophyllum leopoldi at the region's Amazonian edge, and generalists like Aequidens tetramerus and Chaetobranchus flavescens. Almost all are assessed Least Concern — a function of intact habitat rather than abundant study; in the upper basins the fauna is still only patchily known.

Conservation

By global standards the Guianas remain extraordinarily intact. The Guiana Shield is one of the last great blocks of contiguous primary tropical forest on Earth, and across much of the ecoregion the rivers still run essentially undammed and unpolluted through unbroken canopy — which is exactly why the endemic fauna persists. But that intactness is not guaranteed, and the dominant threat has a single name: gold.

Small-scale and illegal gold mining is now the most serious pressure on these rivers. Hydraulically working alluvial and bank deposits dumps suspended sediment into otherwise clear, low-sediment water, raising turbidity and erasing the habitat structure — bank cover, depth, transparency — on which the fish communities are sorted (Mol & Ouboter, 2004; de Mérona et al., 2012). Worse, the trade still relies on elemental mercury to amalgamate gold. That mercury enters the rivers, is converted by bacteria in warm, acidic, organic-rich, low-oxygen shield water — conditions that strongly favour methylation — into neurotoxic methylmercury, and then biomagnifies up the food chain into exactly the large piscivorous fish that people eat. Surveys across the Guianas have repeatedly found elevated mercury in freshwater fish, with the highest burdens in top predators (piscivore muscle measured up to several µg/g, wet weight), and Indigenous and Maroon communities who depend on river fish carry a correspondingly higher dietary exposure (Mol et al., 2001; WWF Guianas mercury report, 2015; PMC mercury studies). The mercury problem is therefore both an ecological and a public-health one, and it does not respect the borders between Guyana, Suriname and French Guiana.

The other classic river impact is impoundment. Dams are few here relative to the Orinoco or the Amazon, but they are consequential where they exist — Suriname's Afobaka (Brokopondo) dam on the Suriname River and the Petit-Saut dam on the Sinnamary in French Guiana each converted a free-flowing shield river into a reservoir, altering downstream flow, oxygen and fish communities and prompting much of the region's impact research (de Mérona et al., 2012). Bauxite and iron mining add their own chemical signatures — iron, aluminium and shifts in pH and conductivity — to affected catchments (Rosales, IUCN-NL, 2003). The encouraging counterpoint is that the baseline is still high: protected areas such as the Central Suriname Nature Reserve and French Guiana's Parc Amazonien guard large intact watersheds, and the conservation task here is less about restoring degraded rivers than about keeping mercury, sediment and dams from spoiling some of the cleanest, most endemic freshwater left in South America.

Sources

1. Guianas — Freshwater Ecoregions of the World (Ecoregion 311; boundaries, topography, endemism, habitats)
2. Fish-habitat relationships in French Guiana rivers: a review (de Mérona, Tejerina-Garro & Vigouroux, Cybium 36(1), 2012)
3. Checklist of the Freshwater Fishes of the Guiana Shield (Vari, Ferraris, Radosavljevic, Funk & Lundberg, 2009; 1,168 species)
4. Updated checklist of the freshwater and estuarine fishes of French Guiana (Le Bail et al.; 416 species, 367 strictly freshwater)
5. Cleithracara maronii, Keyhole cichlid — FishBase (habitat, distribution, biology; after Kullander & Nijssen 1989)
6. Mazarunia: phylogenetic diagnosis and expanded description (Neotropical Ichthyology; Guiana-Shield-endemic dwarf cichlids of the upper Mazaruni)
7. Hydrology in the Guiana Shield and possibilities for payment schemes (Rosales, NC-IUCN / Guiana Shield Initiative, 2003)
8. Small-Scale Gold Mining Related Mercury Contamination in the Guianas (WWF Guianas regional assessment, 2015)
9. Contrasting mercury contamination scenarios and site-specific risk in Suriname gold-mining rivers (PMC / NIH)