A combined approach to establishing the timing and magnitude of anthropogenic nutrient alteration in a mediterranean coastal lake- watershed system

Abstract

Human activities have profoundly altered the global nutrient cycle through Land Use and cover Changes (LUCCs) since the industrial revolution and especially during the Great Acceleration (1950 CE). Yet, the impact of such activities on terrestrial and aquatic ecosystems above their ecological baselines are not well known, especially when considering the response of these systems to the intensity of LUCCs on nutrient cycles. Here, we used a multiproxy approach (sedimentological, geochemical and isotopic analyses, historical records, climate data, and satellite images) to evaluate the role that LUCCs have on nitrogen (n) cycling in a coastal mediterranean watershed system of central chile over the last two centuries. Despite long-term anthropogenic use (agriculture, cattle grazing) in the Matanzas watershed-lake system, these LUCC appear to have had little impact on nutrient and organic matter transfer since the Spanish Colonial period. In contrast, the largest changes in N dynamics occurred in the mid-1970s, driven by the replacement of native forests and grasslands by government-subsidized tree plantations of introduced Monterey pine (Pinus radiata) and eucalyptus (Eucalyptus globulus). These LUCC had major impacts on the transfer of organic matter (which increased by 9.4%) and nutrients (as revealed by an increase in total N) to Laguna Matanzas. Our study shows that the presence of anthropogenic land use/cover changes do not necessarily alter nutrient supply and n availability per se but rather it is the magnitude and intensity of such changes that produce major impact on these processes in these mediterranean watersheds.

Untitled

“…2) with elevated sediment input as indicated by our geochemical proxies (Ti; Fig. 2; Fuentealba et al 2020). Sediment δ 15 N om values from the 14th to 19th centuries were relatively high, probably reflecting N inputs from cow manure and soil particles that typically generate higher sediment δ 15 N om values (Fig.…”

“…The Br/Ti ratios in lake sediment from coastal lakes of central Chile are commonly elevated with high organic carbon content, indicating higher lake productivity (Frugone-Álvarez et al 2017;Fuentealba et al 2020). Similarly, fluctuations in δ 15 N om in lake records have been used as an indicator of changes in paleoproductivity and/or watershed disturbances (Das et al 2009;Torres et al 2012).…”

“…Until the 1970s, the Laguna Matanzas watershed was mostly covered by native forest, and grassland areas were intended for livestock grazing (Fig. 1; Fuentealba et al 2020). From the 1980s onwards, sediment δ 15 N om values decreased, reaching their lowest values in the entire sequence at ca.…”

“…2). The increased lake productivity, the sharp change in δ 15 N om trend, and the decreased sediment input during the last few decades has been synchronous with the replacement of intensive livestock grazing by intensive agriculture and forestry practices (Fuentealba et al 2020). Land use during the colonial period in Lago Vichuquén was characterized by agriculture, cattle ranching, and mining under the Encomienda (feudal) system (Vidal and Ramírez 1985).…”

Volcanism and climate change as drivers in Holocene depositional dynamic of Laguna del Maule (Andes of central Chile – 36° S)

“…S8). Three element ratios have been calculated to describe changes in the redox conditions (Fe/Mn), carbonate (Ca/Ti) and organic productivity (Br/Ti) (Carrevedo et al, 2015;Frugone-Álvarez et al, 2017;Naeher et al, 2013;Moreno et al, 2007;Fuentealba et al, 2020). Thirty-one discrete samples were measured using inductively coupled plasma-optical emission spectrometry (ICP-OES) at the CEBAS-CSIC laboratory in order to analyze the relationship between quantitative elements and XRF semi-qualitative measurements (Fig.…”