The interaction of physical structure and nutrient loading drives ecosystem change in a large tropical lake over 40 years

Jemma M Fadum; Ed K Hall

doi:10.1016/j.scitotenv.2022.154454

The interaction of physical structure and nutrient loading drives ecosystem change in a large tropical lake over 40 years

Sci Total Environ. 2022 Jul 15:830:154454. doi: 10.1016/j.scitotenv.2022.154454. Epub 2022 Mar 10.

Authors

Jemma M Fadum¹, Ed K Hall²

Affiliations

¹ Graduate Degree Program in Ecology, Colorado State University, 102 Johnson Hall, Fort Collins, CO 80523, USA; Department of Ecosystem Science and Sustainability, Colorado State University, Campus Delivery 1476, Fort Collins, CO 80523, USA. Electronic address: jfadum@rams.colostate.edu.
² Graduate Degree Program in Ecology, Colorado State University, 102 Johnson Hall, Fort Collins, CO 80523, USA; Department of Ecosystem Science and Sustainability, Colorado State University, Campus Delivery 1476, Fort Collins, CO 80523, USA; Natural Resource Ecology Laboratory, Colorado State University, Campus Delivery 1499, Fort Collins, CO 80523, USA. Electronic address: ed.hall@colostate.edu.

PMID: 35278553
DOI: 10.1016/j.scitotenv.2022.154454

Abstract

Many lakes across the world are entering novel states and experiencing altered biogeochemical cycling due to local anthropogenic stressors. In the tropics, understanding the drivers of these changes can be difficult due to a lack of documented historic conditions or an absence of continuous monitoring that can distinguish between intra- and inter-annual variation. Over the last forty years (1980-2020), Lake Yojoa (Honduras) has experienced increased watershed development as well as the introduction of a large net-pen Tilapia farm, resulting in a dramatic reduction in seasonal water clarity, increased trophic state and altered nutrient dynamics, shifting Lake Yojoa from an oligotrophic (low productivity) to mesotrophic (moderate productivity) ecosystem. To assess the changes that have occurred in Lake Yojoa as well as putative drivers for those changes, we compared Secchi depth (water clarity), dissolved inorganic nitrogen (DIN), and total phosphorus (TP) concentrations at continuous semi-monthly intervals for the three years between 1979 and 1983 and again at continuous 16-day intervals for 2018-2020. Between those two periods we observed the loss of a clear water phase that previously occurred in the months when the water column was fully mixed. Seasonal peaks in DIN coincident with mixing suggest that an enhanced accumulation of ammonium in the hypolimnion (the bottom layer of a stratified lake) during stratification, and release to the epilimnion (the top layer of a stratified lake) with mixing maintains high algal abundance and subsequently low Secchi depth during what was previously the clear water phase. This interaction of nutrient loading and Lake Yojoa's monomictic stratification regime illustrates a key phenomenon in how physical water column structure and nutrients interact in tropical monomictic lakes. This work highlights the need to consider nutrient dynamics of warm anoxic hypolimnions, not just surface water nutrient concentrations, to understand environmental change in these societally important but understudied ecosystems.

Keywords: Aquaculture; Lake; Nutrient cycling; Stratification; Trophic state; Tropical.

MeSH terms

Ecosystem*
Environmental Monitoring
Eutrophication
Lakes* / chemistry
Nitrogen / analysis
Nutrients / analysis
Phosphorus / analysis
Water

Substances

Water
Phosphorus
Nitrogen