UWI students engaged in learning about aquatic biodiversity. (Photo by M. Alkins-Koo)

Research in the Department of Life Sciences has recently focussed on identifying impacts of human activities on rivers of Trinidad and Tobago. Local rivers are being severely degraded particularly by pollution in areas of high population density or where extractive industries (oil, minerals) are located. Other types of impact include those due to habitat change, introduction of exotic species, and exploitation.

There are increasing stresses in urbanised areas such as southwest Tobago, northwest, west central and south Trinidad where sewage and domestic run-off (grey water) cause varying degrees of nutrient enrichment. The lower courses of some rivers along the East-West corridor studied by Master’s student Floyd Lucas are almost dead except for sewage fungus and fly larvae.

Rivers: Dead or Alive? Lower St Joseph River (above) and Guanapo River (left).

As PhD student Sharda Surujdeo-Maharaj has demonstrated, rivers are contaminated with heavy metals from various industrial and commercial activities but surprisingly, also from sources such as sewage contamination (zinc) and vehicle emissions (lead). Even in ‘pristine’ sites with no visible wastes, she found river sediments contaminated with lead, probably caused by deposition of road dust and vehicle emissions, and washing of cars in rivers.

Biodiversity can be negatively affected in a variety of ways. For example, toxic substances such as industrial chemicals, pesticides, heavy metals and oil may have direct deadly effects or sub-lethal effects on organisms, e.g. making them more susceptible to other stresses like parasites. Sediments from quarry washings and construction run off smother bottom-dwelling organisms and reduce food availability.

Since some freshwater fish and shrimp migrate during their life cycles, pollution at any point on the river can affect them in that river system. This is evident in the studies of Dawn Phillip and Wayne Rostant where mountain mullet or freshwater shrimp are no longer found in upstream tributaries of the Caroni River as in the past since their natural migrations are effectively ‘blocked’ by the polluted lower river.

Even attempts at flood-protection (dredging, channelisation and construction of embankments) decrease species richness by simplifying habitats, increasing disturbance, decreasing substrate stability, smothering, and depriving aquatic organisms of their natural energy and nutrient sources from the surrounding vegetation.

Many rural rivers are in fair condition and have extensive bankside vegetation cover which appears to be a significant factor producing good water quality and healthy aquatic ecosystems. Any clearance for agriculture can result in soil erosion and washing of sediment and nutrient into streams.

Riverbed mining for construction aggregate was particularly evident in Tobago and resulted in severe disruption of these rivers. Tractors excavate large holes in the riverbed leaving the rubble bare and unstable and produce high levels of turbidity.

Introduced species can also be a threat to native organisms and over the years, several exotic species have become established in local rivers. These include Tilapia, several aquarium fish, the Tiger Prawn Macrobrachium rosenbergii, two species of invasive Asian snails, and aquatic plants such as Brazilian Elodea (pond weed). The ecological impacts of these introductions on native species are as yet unknown and require investigation.

Exploitation of fish, crabs and freshwater shrimp for food seems to occur largely on a subsistence level but it is not quantified by regulatory agencies so it is difficult to make an accurate assessment. The manicou crab is one notable delicacy and they are caught easily when they ‘run’ during and after heavy rain showers along mountain roads. Luke Rostant’s research shows that these crabs mature at fairly large sizes so capture of large females bearing young can jeopardise the survival of future populations.

Fishing for teta for the aquarium trade takes place in many lowland rivers and raises public health issues since we have seen children wading and snorkelling about in water contaminated by sewage.

One mechanism to assess change is biological monitoring, the use of organisms present in rivers to indicate the nature of the conditions in which they live. Our research recommends the use of rapid assessment techniques based on a combination of fish and bottom-dwelling invertebrates. As part of her PhD work, Lorraine Maharaj is producing standardised field and laboratory protocols for biomonitoring of local rivers using invertebrates.

The studies reported here form the basis of a network for development of a future monitoring system for aquatic biodiversity assessment which can be implemented by the regulatory agencies in collaboration with UWI researchers.

However, this can only tell us what the conditions are – there is still the issue of the root cause. The management of local aquatic ecosystems rests heavily on land management and sustainable land use practices within the watershed unit. A holistic approach must be taken.

Cumulative effects of a variety of influences must be assessed and therefore need multidisciplinary, intersectoral collaboration.

Identification of priority areas and issues for management must be followed by development of action plans for implementation. For example, delineation of ‘hot-spot’ geographic areas needing immediate clean-up action, implementation of legislation such as the Water Pollution Rules, monitoring of invasive species, rehabilitation and restoration of degraded ecosystems need to be addressed urgently.

If rivers are considered from a multi-disciplinary perspective, they must include humans as an integral component. Firstly, public education and awareness programmes are essential to encourage sustainable behaviour with respect to use of rivers and their associated ecosystems.

And secondly, further investigation of the socioeconomic systems dependent upon water resources, aquatic biodiversity and other services provided by rivers, is urgently needed especially focusing on the relationships between these resources and human health and wellbeing.

[Adapted from a paper presented at the Second Caribbean Environmental Forum CEF-2, May/June 2004]