Counting the Hidden Energy That Keeps an Estuary Alive

On the southern coast of Santa Catarina in Brazil, the Araranguá River meets the Atlantic Ocean through a narrow, shifting estuary that few outside the region could name. Yet this modest water body, formed where the Itoupava and Mãe Luzia rivers converge, sustains fisheries, farmland, tourism and a coastal culture that has grown up around it for generations. A new study published in the journal Estuarine Management and Technologies asks a deceptively simple question about this place: how much energy does nature actually invest in keeping the estuary alive? The answer, expressed in a currency most people have never heard of, offers a fresh way to argue for conservation in terms that policymakers can weigh against economic pressure.

The research applies a framework known as emergy analysis, a method developed by the ecologist Howard T. Odum in the latter half of the twentieth century. Emergy, short for embodied energy, is a form of environmental accounting that traces every flow of energy and material in a system back to the solar energy originally required to produce it. Because sunlight drives wind, rain, rivers, tides and the growth of every living thing, emergy provides a common denominator for comparing phenomena that would otherwise resist comparison. A cubic meter of river water, a tonne of sediment and a scenic view of dunes can all be expressed in the same units, allowing analysts to add up the total environmental subsidy that a landscape receives each year.

The study area is defined with unusual precision. The Araranguá estuary stretches roughly thirty kilometers, covers 4.63 million square meters, and drains a basin of just over three billion square meters that spans fourteen municipalities. The combined rivers carry an average of 1.67 billion cubic meters of water to the sea each year. That catchment has long been shaped by human hands: rice paddies line the floodplains, livestock graze the uplands, and coal mining has left its mark on soils and waterways. Decades of such activity have produced soil erosion, water degradation and ecological decline, making the question of what the estuary needs to function all the more urgent.

To build the energy model, the researchers combined expert knowledge, published literature and field observations into a comprehensive systems diagram drawn in Odum’s energy systems language. This visual notation uses a set of specialized symbols to represent the structure and function of an ecosystem: sources of energy, storage compartments, interaction points where flows combine, and consumers that transform energy from one form to another. The diagram aggregates comparable components and processes, and it can be disaggregated whenever a specific question demands finer detail. From this model, the team constructed an emergy assessment table containing equations that calculate annual energy flows in joules, material flows in grams, and monetary flows in reais, converting each into emergy terms.

The headline result is that the total environmental energy sustaining the Araranguá River estuary amounts to approximately 2.07 million emReais per year, a synthetic unit that expresses emergy in monetary equivalents. Of this total, the resources directly sustaining the estuary account for 1.04 million emReais annually. The two tributaries dominate the budget: the Itoupava and Mãe Luzia rivers together contribute the equivalent of 947,000 reais per year, while direct rainfall over the estuary adds a further 78,000 reais. Tides, which pulse twice daily through the inlet, form another essential driver of the system, mixing marine and freshwater and shaping the sandy barrier, islands and rocky outcrop that give the estuary its character.

The researchers evaluated the ecosystem’s contribution from two complementary perspectives. The first considers the system-level flows of energy and materials that physically sustain the estuary’s structure. The second addresses the environmental costs associated with the estuary’s very existence, including its aesthetic value, which residents and tourists alike appreciate. The rocky outcrop near the mouth, fringed with dunes, deflects the channel northward and lends the coastline a scenic quality that underpins local tourism. Although the study did not directly quantify the region’s economic activities, those activities are demonstrably built upon the renewable energy foundation that the analysis measures, from the freshwater that irrigates rice fields to the landscapes that draw visitors.

Underpinning the method is a set of theoretical principles drawn from systems theory and thermodynamics. The principle of maximum power holds that systems which capture and use energy efficiently tend to prevail over competitors. Energy hierarchy theory recognizes that transformations of energy occur in ordered steps, with each conversion losing available energy as it moves up the chain from sunlight to vegetation to animal life to human economies. The pulsating paradigm describes how ecosystems grow and contract in cycles rather than settling into permanent equilibrium, a recognition of systemic impermanence driven by the constant exchange of energy between living and nonliving components. Concepts such as thermodynamic entropy and irreversibility reinforce the point that environmental change is often one-way: once energy has flowed through a system and degraded, it cannot be recovered.

What makes emergy analysis powerful is precisely its universality. By expressing every input in common terms, it allows a decision-maker to compare the value of a wetland’s water purification with the revenue from a housing development, or the contribution of a river’s flow with the cost of a dredging project. In the case of the Araranguá estuary, the analysis identified which energy sources are most critical to maintaining the system’s structure, providing an evidence base for allocating resources and designing conservation strategies. The authors argue that this kind of holistic accounting can inform sustainable public policies while minimizing negative impacts, and that effective communication and environmental education are essential for conveying such complex processes to the audiences who must act on them.

The study also highlights a governance challenge that extends beyond the science. The Mãe Luzia and Itoupava rivers traverse multiple municipalities before they ever reach the estuary, meaning that upstream land use in one community shapes the energy budget of a coastal system in another. Integrated management therefore requires regional cooperation among all stakeholders, a political task that no energy diagram can accomplish by itself. The authors note that recognizing landscape-forming elements such as rivers, tides and rainfall as genuine contributors to economic and ecological wellbeing remains one of the hardest steps in environmental management, particularly where interpretive frameworks for authenticity in research and policy are still lacking.

The findings arrive at a moment when coastal Brazil faces mounting pressure from real estate speculation, which the authors explicitly identify as a threat to the estuary’s ecological and economic value. By attaching a defensible emergy figure to the natural flows that sustain the Araranguá system, the research offers local governments and communities a quantitative argument for preservation that can stand alongside conventional economic assessments. The estuary’s annual contribution, valued at roughly 1.04 million emReais in direct resources and 2.07 million emReais when aesthetic value is included, is not merely an abstract number. It is a measure of the sun-driven work of rivers, rain and tides that has quietly built and maintained one of southern Brazil’s most important coastal landscapes, and a reminder that the true cost of development is often counted in energy that no market has ever priced.

Subject of Research: Emergy analysis of renewable energy contributions to the Araranguá River estuary in southern Brazil

Article Title: Is it possible to evaluate the energy contributions of an estuary?

Article References: Carvalho-Junior, O. D. O., & D’Aquino, C. A. (2025). Is it possible to evaluate the energy contributions of an estuary?. Estuarine Management and Technologies, 2, 5-7. https://doi.org/10.3897/emt.2.151294

Image Credits: AI Generated

DOI: 10.3897/emt.2.151294

Keywords: emergy analysis, estuary, Araranguá River, ecosystem services, Odum energy systems, environmental accounting, Santa Catarina, conservation policy, renewable energy flows, estuarine management, possible, evaluate