Eastern edge of Macuruco Lagoon, blackwater tributary of the Rio Orinoco, Venezuela

_st place in Biotope Aquarium Design Contest 2022

Volume of aquarium: 360 Liters

Dimensions of aquarium: 122x46x64 cm

List of fishes: Hemigrammus rhodostomus, Apteronotus leptorhynchus, Ancistrus macrophthalmus, Pomacea bridgesii, Palaemonetes sp.

List of plants: Lemna minor, Pistia stratiotes L.

Description of Decorations and Substrate: The pale sand of the river bottom is emulated using CaribSea Supernaturals “Crystal River” substrate. The fallen branches and driftwood that accumulate in the creek bends [1, p. 110] [2, p. 3] are created using spider wood, manzanita branches, and driftwood. The rocks that occur along the bottom, said to be granite with a pinkish color [3, pp. 535, 560], are represented using river rocks sourced from the Canadian Shield (parts of which have similar geology to the Guiana Shield) by Maynooth Granite.

The leaf litter essential to the substrate [1, p. 110] [2, p. 3] is formed using a variety of botanicals, with emphasis given to leaves and leaf shapes that occur in the region such as from the guava tree (Psidium guajava) [4]. The substrate is also decorated with botanicals including seed pods found in the region from trees of the genera Sterculia [5], Kielmeyera [5], and Jacaranda [1, p. 185] from Tannin Aquatics. Mulm, the combination of decaying organic materials vital to the environment and feeding of several of these species, is generated naturally in both the substrate and water column through biological activity and the decomposition of these elements.

Description of Equipment: Plumbing: Durso-style standpipe overflow (1” ID PVC), sump tank Aqueon 20 gallon long aquarium (dimensions 77.5×33.3×33.0 cm) with added acrylic baffles, return pump Sicce Syncra SDC 6.0 at 33% power, additional hosing silicone tubing with ID of 3/4″ (return line) and 1” (overflow line)
Heater: Hygger 500W Aquarium Heater, UV sterilizer: AquaUV 8W Advantage 2000, Lighting: Fluval Aquasky (display tank) and Fluval Plant 3.0 (sump) programmable LED light bars

Water Parameters: Temperature is 26° C, pH is 6.6, GH is 5, KH is 3 or less, ammonia (NH4+) and nitrite (NO2-) are 0ppm, nitrate (NO3-) is 5ppm or less

Additional Info: The Durso-style standpipe creates aeration in the water, such as would be provided by more turbulent waters upstream in nature, providing oxygenation for the comfort of the fish while allowing for calmer flow in the main aquarium to mimic the biotope of the lagoon. Lack of carbon dioxide for the plants due to aeration is not a problem, as all plants are floating or (in the sump) growing emersed.

The sump tank is mostly occupied by a large refugium filled with fast-growing plants, including Rotala rotundifolia, Ceratophyllum demersum, Lemna minor, and Echinodorus bleherii. In addition to shedding organics into the water as would be done by plants further upstream in the natural biotope, these plants remove contaminants and nitrates from the water, in a modification of the Walstad method [6]. By removing a handful or two of plants each week as nutrient export, more are allowed to grow, removing more waste from the water column and minimizing water changes to approximately 10 percent every 3 months (mostly to remove excess mulm) while maintaining stable parameters and a healthy, natural environment. The refugium is also home to a second colony of Palaemonetes sp., providing additional detritus breakdown. The lights in the sump tank are kept on a reverse day/night cycle from the main tank, so that all creatures can enjoy a normal day/night cycle but plants are always active somewhere in the system to purify and oxygenate the water.

Additional leaf litter and botanicals are added as needed, about every month or two, to replace those that decay.

While Apteronotus leptorhynchus can predate on Palaemonetes spp., an abundance of hiding places due to the leaf litter combined with regular feeding of A. leptorhynchus restricts the predation in the tank to no more than an occasional fry, ensuring that the shrimp colony remains stable or growing as births outpace deaths.

The UV sterilizer serves to keep the bacterial count in the water low, preserving the health of the fish by mimicking the lack of bacterial diseases present in natural blackwater environments due to the low pH [7].

Trace elements are provided by a substrate in the sump consisting of 9 kg Ecosystem Freshwater Miracle Mud, half of which is replaced annually. This substrate, by remaining undisturbed in the sump, also provides a hypoxic environment in which denitrification by bacteria can occur to remove excess nitrates [6, pp. 63-64, 127-129].

Aquarium video:

Description of the Area Surrounding the Biotope: The Orinoco River is the fourth largest river in the world by discharge volume, and its drainage basin covers most of Venezuela and part of Columbia [8]. The mouth of the Orinoco was discovered by Europeans early, being visited by Christopher Columbus during his third voyage, and the lower reaches of the river through the plains of Venezuela and Colombia have become increasingly urbanized and industrialized since the middle of the 20th century. In contrast, the upper reaches of the river, deep in the rainforest at the periphery of the Guiana Shield, have been much harder to explore, with the source of the river only being mapped in 1951 [9].

The highland rainforests, with their many hills, streams, and rivers, are home to many mysteries. Some boulders along the rivers are decorated with petroglyphs, ancient designs carved deeply into the granite, for which not even the current indigenous peoples know the meaning or origin [3, pp. 535, 538-9]. Large, tabletop mountains known as tepuis rise high above the forest, harboring their own climates and unique species of plants and animals in sheltered highlands isolated by rocky cliffsides [10]. Banana trees grow wild alongside the watercourses, a relic of the early Spanish explorers and settlers [3, pp. 543-4]. The varied biomes and abundant natural resources contribute to the Guiana Shield’s incredible biodiversity, with over 4,000 vertebrate animal species and 20,000 plant species, an estimated 40% of which are endemic [11].

In these upper reaches of the river, the main Orinoco River is joined by many tributaries, ranging from tiny seasonal streams to respectable rivers in their own right. One of the smaller tributaries, the Caño Macuruco, joins the Orinoco just upstream from the confluence of the Orinoco and Ventuari Rivers at the Santa Barbara Rapids. Just over 2 km from its own confluence with the Orinoco, a wide bend in Caño Macuruco forms a calmer, wider section known as Macuruco Lagoon at 3°55’20″N 67°0’10″W. Surrounded by riparian forest, the lagoon lies in the middle of a periodically-flooded alluvial plain with a silt-sandy texture. While the western bank is bounded by a series of 1-2 m high terraces, the eastern edge is bordered by a strip of forest about 7 m high and flooded to a depth of up to 2.5 m [1, p. 51] before giving way to a patch of savannah further east.

Description of the Underwater Landscape of the Biotope: Macuruco Lagoon covers several hectares of black water, abundant with leaf litter and decomposing driftwood. These become lodged among the roots of the trees along the edges of the lagoon, forming dense tangles as they break down into a variety of floating organic compounds and bits of detritus. Though dark in color and heavily shaded, the water is transparent for over 2 m, allowing some visibility [1, pp. 93, 110]. Plants are almost entirely absent from the riverbed, but floating plants congregate in the calmer waters along the edges [12]. Amongst the lightly-colored sandy bottom [2, p. 3], branches, and leaf litter, explorers have discovered frequent rocks along the riverbed, described as coarse-to-medium granite with some pinkish colors [3, pp. 535, 560].

Due to the acidic nature of the environment and a scarcity of calcium ions, many are surprised to find that shelled invertebrates such as shrimp and snails populate this environment. Yet not only do studies indicate that shrimp can be found year-round even with an acidic pH approaching 4.0 [13], 2.5 hours of sampling in Macuruco Lagoon yielded over 200 shrimp from the Palaemonidae family [1, p. 110], as well as snails of the Pomacea genus [1, p. 99]. These and a variety of aquatic and semiaquatic insects provide biological decomposition and combine with the plentiful floating organics to provide a rich food source for the diverse species of fish that inhabit the area.

With both invertebrates and vegetable matter as readily available food, a wide variety of fish are present in the lagoon, especially during the wet season. A plethora of characids, including Hemigrammus rhodostomus, dart back and forth amongst the driftwood, which is in turn home to algae and bacterial colonies fed on by loricariids such as Ancistrus macrophthalmus. Even on the darkest of nights, there is plenty of activity, as gymnotiformes such as Apteronotus leptorhynchus navigate and hunt for prey using electrical fields generated by their own bodies.

Description of the Habitat Parameters: The water is dark and transparent up to 210 cm, with a conductivity between 6.6 and 7.2 μS/cm [1, pp. 93, 203]. Temperature ranges from 24°C to 28°C, but may be less after a downpour. The pH can vary with the seasons, rain, and exact location from close to 4.0 to nearly 7.0 [14], with 5.0 as an approximate average [2, p. 3].

List of Fishes and Invertebrates Occurring in the Nature Biotope: Fish [15]: Asiphonichthys condei, Hemigrammus bellottii, Hemigrammus gr. micropterus, Hemigrammus stictus, Hyphessobrycon sp., Moenkhausia colletti, Moenkhausia lepidura, Paracheirodon innesi, Cyphocharax spilurus, Carnegiella marthae, Nannostomus eques, Nannostomus marilynae, Nannostomus unifasciatus, Microsternarchus bilineatus, Belonion dibranchodon, Apistogramma cf. iniridae, Dicrossus filamentosus, Pterophyllum altum, Satanoperca daemon [1, pp. 206-212]; Hemigrammus rhodostomus, Apteronotus leptorhynchus, Ancistrus macrophthalmus [16]
Invertebrates: Palaemonetes spp., Euryrhynchus amazoniensis [1, p. 110]; Pomacea sp., Notonectidae sp., Polymitarcidae sp. [1, p. 99]

List of Plants Found in the Nature Biotope: Though shade and dark water prevent any benthic plants from being present, calmer regions such as Macuruco Lagoon can exhibit several floating plant species, including Lemna minor (cosmopolitan), Phyllanthus fluitans, and Pistia stratiotes (cosmopolitan) [12].

Threats to the Ecology of the Biotope: Though the region has been largely pristine until recent decades due to the difficulty of accessing it, the ecology is currently threatened by illegal mining. Additionally, overfishing may be causing the decline of some species, whether for subsistence, for export (of ornamental species), or as trophies for tourist sport fishermen (particularly the brightly-colored Peacock Bass [genus Cichla]). While these fishing activities can be a valuable way to provide income for local residents and keep them invested in the long-term health of the river system, pursuing them too aggressively could destroy the very species on which they depend. In addition to the eradication of illegal mining, more structured seasons and quotas are needed to ensure that fishing activities are sustainable and local communities can share fairly in the river’s bounty. [1, pp. 141-146]

Another threat to the ecology is simply a lack of understanding. A scientific survey of the area in 2003 discovered 13 previously unknown species of fish (as well as a new species of crustacean) [1, p. 18], but was only able to describe many of the invertebrates of the area by family. The freshwater shrimp have been “practically unknown” [17], and even some of the fish exported from the region for the ornamental trade are frequently misidentified [18]. Subsequent expeditions have continued to yield valuable samples and the discovery of new species [19], with one researcher describing the region as a “poorly studied biodiversity hotspot.” [20] Clearly, the many types of animals that inhabit this region and their interactions are not well understood, and it may prove difficult to balance competing demands on the river with an incomplete understanding of the existing ecology.

Sources of Information:
[1] C. A. Lasso, J. Celsa Señaris, L. E. Alonso and A. L. Flores, Eds., Evaluación Rápida de la Biodiversidad de los Ecosistemas Acuáticos en la Confluencia de los ríos Orinoco y Ventuari, Estado Amazonas (Venezuela), Vols. RAP Bulletin of Biological Assessment, Volume 30, Washington, D.C.: Conservation International, 2006. Accessible at https://www.researchgate.net/publication/277011628_Evaluacion_Rapida_de_la_Biodiversidad_de_los_Ecosistemas_Acuaticos_de_la_Confluencia_de_los_Rios_Orinoco_y_Ventuari_Estado_Amazonas_Venezuela
[2] E. Ruiz, “Pterophyllum altum biotopes,” Fintastic Newsletter, no. 23, pp. 1-5, August 2009. Accessible at http://www.theangelfishsociety.org/newsletters/2009_Aug_v23.pdf
[3] C. B. Hitchcock, W. H. Phelps Jr. and F. A. Galavis, “The Orinoco-Ventuari Region, Venezuela,” The Geographical Review, vol. 37, no. 4, pp. 525-566, October 1947. Accessible at https://doi.org/10.2307/211185
[4] S. Sua and D. Cárdenas, “Herbario Amazónico Colombiano,” 2021.
[5] J. Solomon and H. Stimmel, “Tropicos Specimen Data,” Missouri Botanical Garden, 2021.
[6] D. Walstad, Ecology of the Planted Aquarium, 3rd ed., Echinodorus Publishing, 2013.
[7] P. Rockstroh, “Blackwater Streams and Blackwater Fish,” 28 July 2018. [Online]. Available: https://www.exoticaesoterica.com/new-blog-1/2018/7/28/blackwater-streams-and-fish.
[8] M. Brys, “Orinoco River – WorldAtlas,” WorldAtlas, 9 August 2021. [Online]. Available: https://www.worldatlas.com/rivers/orinoco-river.html.
[9] N. W. E. contributors, “Orinoco River,” New World Encyclopedia, 4 January 2019. [Online]. Available: https://www.newworldencyclopedia.org/p/index.php?title=Orinoco_River&oldid=1016878.
[10] R. Sears, “Tepui,” World Wildlife Foundation, [Online]. Available: https://www.worldwildlife.org/ecoregions/nt0169. [Accessed January 2022].
[11] Amazon Conservation Team, “The Guiana Shield: One of the Last Wild Places on Earth,” 2021. [Online]. Available: https://www.amazonteam.org/maps/guiana-shield/.
[12] Whitepine, “Orinoco River Basin Biotope Aquarium,” PlantedTank.net, 3 April 2006. [Online]. Available: https://www.plantedtank.net/threads/orinoco-river-basin-biotope-aquarium.28816/#post-249778.
[13] S. Fellman, “On the Trail of the Amazonian Shrimp,” Tannin Aquatics, 13 October 2019. [Online]. Available: https://www.tanninaquatics.com/blogs/the-tint-1/on-the-trail-of-the-amazonian-shrimp.
[14] J. Thornes, “Variability in Specific Conductance and pH in the Casiquiare–Upper Orinoco,” Nature, vol. 221, pp. 461-462, 1969.
[15] “Seriously Fish Knowledge Base,” Seriously Fish, [Online]. Available: https://www.seriouslyfish.com/knowledge-base/.
[16] B. Millen, “Ichthyology Collection – Royal Ontario Museum,” Royal Ontario Museum, 2019. [Online]. Available: https://www.gbif.org/occurrence/1211721288, https://www.gbif.org/occurrence/1211721367, https://www.gbif.org/occurrence/1211720595.
[17] G. Rodriguez, “Fresh-Water Shrimps (Crustacea, Decapoda, Natantia) of the Orinoco Basin and the Venezuelan Guayana,” Journal of Crustacean Biology, vol. 2, no. 3, pp. 378-391, 1982.
[18] H. Hagenson and L. Jamne, “Ancistrus macrophthalmus,” [Online]. Available: https://www.loricariidae.info/ancistrus-macrophthalmus.
[19] N. K. Lujan, “The Guiana Shield,” [Online]. Available: https://www.loricariidae.org/guiana-shield.
[20] N. K. Lujan, “2003 Expedition to the Confluence of the Orinoco and Ventuari Rivers, Amazonas State, Venezuela,” [Online]. Available: https://www.loricariidae.org/nkl03.