Min-Max principle of entropy production with time in aquatic communities

Entropy production in a fluid-solid system far from thermodynamic equilibrium

The terminal orientation of a rigid body in a moving fluid is an example of a dissipative system, out of thermodynamic equilibrium and therefore a perfect testing ground for the validity of the maximum entropy production principle (MaxEP). Thus far, dynamical equations alone have been employed in studying the equilibrium states in fluid-solid interactions, but these are far too complex and become analytically intractable when inertial effects come into play. At that stage, our only recourse is to rely on numerical techniques which can be computationally expensive. In our past work, we have shown that the MaxEP is a reliable tool to help predict orientational equilibrium states of highly symmetric bodies such as cylinders, spheroids and toroidal bodies. The MaxEP correctly helps choose the stable equilibrium in these cases when the system is slightly out of thermodynamic equilibrium. In the current paper, we expand our analysis to examine i) bodies with fewer symmetries than previously reported, for instance, a half-ellipse and ii) when the system is far from thermodynamic equilibrium. Using two-dimensional numerical studies at Reynolds numbers ranging between 0 and 14, we examine the validity of the MaxEP. Our analysis of flow past a half-ellipse shows that overall the MaxEP is a good predictor of the equilibrium states but, in the special case of the half-ellipse with aspect ratio much greater than unity, the MaxEP is replaced by the Min-MaxEP, at higher Reynolds numbers when inertial effects come into play. Experiments in sedimentation tanks and with hinged bodies in a flow tank confirm these calculations.

Impact assessment of excess discharges of organics and nutrients into aquatic systems by thermodynamic entropy calculation

In this study, a method was proposed for calculating the thermodynamic entropy increase ΔS in a water body after receiving excess discharge of organics and nutrients in order to quantitatively assess the impact of pollutants discharge on aquatic systems. The enthalpy change was evaluated using the standard thermodynamic data according to the possible chemical and/or biochemical reactions such as organic oxidation, nitrification/denitrification, and phosphorus precipitation, for the recovery of water quality to the background level. A series of equations were established for calculating the ΔS associated with the decomposition or removal of TOC, NO(3)-N, NO(2)-N, NH(3)-N and TP. The values of ΔS corresponding to unit mass (per g) of these pollutants were calculated as 54.0 kJ/K, 2.91 kJ/K, 10.01 kJ/K, 28.51 kJ/K and 2.81 kJ/K, respectively. Besides, the applicability of the proposed method was proved by a scenario analysis regarding effluent quality control and surface water quality protection in China.

Variación temporal de la condición trófica del lago del Parque Norte, Medellín (Antioquia), Colombia

Con el propósito de evaluar la variación temporal del estado trófico y la condición húmica del lago del Parque Norte (LPN), Medellín (Antioquia, Colombia), se realizaron muestreos quincenales entre marzo y agosto de 2004, en la mitad de la columna de agua en una estación de muestreo. Se utilizaron los índices de estado trófico de Carlson (IETC), el mismo modificado por Toledo et al. (IETM), el índice de estado trófico basado en la clorofila a activa (IET Cl. a) y el coeficiente de atenuación (k) propuesto por Poole y Atkins. Los resultados obtenidos muestran que el IETM describió mejor el estado trófico del lago. Los bajos valores de los coeficientes de variación (CV) de dicho índice [IETM(DS), CV = 5,9%; IETM(PT), CV = 19,5%; IETM(Cl. a), CV = 13,5%); IETM(promedio), CV = 8,2%] indican que el LPN no presentó una variación temporal significativa en su condición trófica durante el tiempo de muestreo y tendió a permanecer en una condición de eutrofia. El coeficiente de atenuación (k = 2,1 m-1; CV = 21,7%) corrobora esta condición. Por su parte, el IET Cl. a situó el lago en una categoría que osciló entre β mesotrófica y α eutrófica, constatándose nuevamente que existe fuerte tendencia hacia la eutrofia. Aunque desde los puntos de vista físico y químico el cuerpo de agua es poco variable, el fósforo total (PT), el nitrógeno total (NT) y la clorofila a activa (Cl. a) sí variaron temporalmente de manera significativa (PT, CV = 53%; NT, CV = 85%; Cl. a, CV = 63%), por lo que se constituyen en buenos referentes para realizar una descripción aproximada de la dinámica de los ensamblajes biológicos de este cuerpo de agua. Por último, se obtuvo alta variación del contenido de ácidos húmicos (CV = 63%), y su valor medio (20,4 g l-1) permitió ubicar al LPN en la categoría mesohúmica.

Ecosystem functioning and maximum entropy production: a quantitative test of hypotheses

The idea that entropy production puts a constraint on ecosystem functioning is quite popular in ecological thermodynamics. Yet, until now, such claims have received little quantitative verification. Here, we examine three 'entropy production' hypotheses that have been forwarded in the past. The first states that increased entropy production serves as a fingerprint of living systems. The other two hypotheses invoke stronger constraints. The state selection hypothesis states that when a system can attain multiple steady states, the stable state will show the highest entropy production rate. The gradient response principle requires that when the thermodynamic gradient increases, the system's new stable state should always be accompanied by a higher entropy production rate. We test these three hypotheses by applying them to a set of conventional food web models. Each time, we calculate the entropy production rate associated with the stable state of the ecosystem. This analysis shows that the first hypothesis holds for all the food webs tested: the living state shows always an increased entropy production over the abiotic state. In contrast, the state selection and gradient response hypotheses break down when the food web incorporates more than one trophic level, indicating that they are not generally valid.