water Review Ecological Degradation of the Yangtze and Nile Delta-Estuaries in Response to Dam Construction with Special Reference to Monsoonal and Arid Climate Settings Zhongyuan Chen 1,*, Hao Xu 2 and Yanna Wang 1 1 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China; [email protected] 2 Department of Geography and Spatial Information Techniques, Ningbo University, Ningbo 315211, China; [email protected] * Correspondence: [email protected] Abstract: This study reviews the monsoonal Yangtze and the arid Nile deltas with the objective of understanding how the process–response between river-basin modifications and delta-estuary ecological degradation are interrelated under contrasting hydroclimate dynamics. Our analysis shows that the Yangtze River had a long-term stepwise reduction in sediment and silicate fluxes to estuary due to dam construction since the 1960s, especially after the Three Gorges Dam (TGD) closed in 2003. By contrast, the Nile had a drastic reduction of sediment, freshwater, and silicate fluxes immediately after the construction of the Aswan High Dam (AHD) in 1964. Seasonal rainfall in the mid-lower Yangtze basin (below TGD) complemented riverine materials to its estuary, but little was available to the Nile coast below the AHD in the hyper-arid climate setting. Nitrogen (N) and phosphate (P) fluxes in both river basins have increased because of the overuse of N- and P-fertilizer, land-use changes, Citation: Chen, Z.; Xu, H.; Wang, Y. urbanization, and industrialization. Nutrient ratios (N:P:Si) in both delta-estuaries was greatly Ecological Degradation of the Yangtze altered, i.e., Yangtze case: 75:1:946 (1960s–1970s), 86:1:272 (1980s–1990s) and 102:1:75 (2000s–2010s); and Nile Delta-Estuaries in Response and Nile case: 6:1:32 (1960s–1970s), 8:1:9 (1980s–1990s), and 45:1:22 (2013), in the context of the to Dam Construction with Special optimum of Redfield ratio (N:P:Si = 16:1:16). This led to an ecological regime shift evidenced by Reference to Monsoonal and Arid Climate Settings. Water 2021, 13, 1145. a long-term change in phytoplankton communities in the Yangtze estuary, where silicious algae https://doi.org/10.3390/w13091145 tended to lose dominance since the end of the 1990s, when more toxic dinoflagellates began to emerge. In the Nile estuary, such a regime shift was indicated by the post-dam dramatic reduction in Academic Editor: Bommanna zooplankton standing crop and fish landings until the early 2000s when biological recovery occurred Krishnappan due to nutrient inputs from anthropogenic sources. Although the Yangtze had higher human impacts than the Nile in terms of population, industrialization, and fertilizer application, N concentrations Received: 18 March 2021 in the Nile estuarine waters surpassed the Yangtze in recent decades. However, eutrophication Accepted: 19 April 2021 in the Yangtze estuary is much more intensive than in the Nile, leading to the likelihood of its Published: 21 April 2021 estuarine water becoming more acidic than ever before. Therefore, ecological degradation in both delta-estuaries does not follow a linear trajectory, due not only to different climate dynamics but Publisher’s Note: MDPI stays neutral also to human forcings. The comparative insights of this study should be incorporated into future with regard to jurisdictional claims in integrated coastal management of these two important systems. published maps and institutional affil- iations. Keywords: altered nutrient ratio; riverine flux; bio-transformation; river-basin-scale; post-dam consequences Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 1. Introduction This article is an open access article distributed under the terms and River basins have been extensively modified by intensive human activities over the conditions of the Creative Commons last half-century, causing severe environmental degradation globally in delta-estuary sys- Attribution (CC BY) license (https:// tems [1,2]. Overuse of bio-chemical fertilizers, water transfers, urbanization, deforestation, creativecommons.org/licenses/by/ and industrialization in river basins have increasingly degraded healthy habitats on which 4.0/). human society depends. Dam construction in river basins has considerably altered the Water 2021, 13, 1145. https://doi.org/10.3390/w13091145 https://www.mdpi.com/journal/water Water 2021, 13, 1145 2 of 10 which human society depends. Dam construction in river basins has considerably altered the delivery of riverine materials to the sea, mainly sediment, water, nitrogen (N), phos- Water 2021, 13, 1145phate (P), and silicate (Si). As a result, the health of delta-estuarine systems remains at risk 2 of 10 [1,3]. Using China’s Yangtze delta and Egypt’s Nile delta as a comparative example, the objective of thisdelivery paper was of riverine to highlight materials how to riverine the sea, mainly delivery sediment, processes water, and nitrogen related (N), mech- phosphate anisms undergo(P), change and silicate and how (Si). Asdelta a result,-estuarine the health responses of delta-estuarine occur ecologically, systems remainsgiven their at risk [1,3]. Using China’s Yangtze delta and Egypt’s Nile delta as a comparative example, the contrasting geographic setting in terms of climate change and anthropogenic intervention objective of this paper was to highlight how riverine delivery processes and related mecha- (hereafter dualnisms forcings). undergo Although change both and how the delta-estuarineYangtze and Nile responses delta- occurestuaries ecologically, have some given their similarities in processcontrasting-response geographic of environmental setting in terms degradation, of climate change there and are anthropogenic also differences intervention that inform how(hereafter human dualimpacts forcings). affect Although these two both important the Yangtze deltaic and systems. Nile delta-estuaries have some The humansimilarities population in process-response is a key forcing of in environmental modifying the degradation, land surface there of are both also river differences basins. There werethat informabout how200 million human impactspeople affectin the these Yangtze two important basin in the deltaic 1960s systems. and, since then, it has increasedThe to human about 400 population million is (http://www.stat a key forcing in modifyings.gov.cn/tjsj/ndsj/ the land ( surfaceaccessed of bothon river 2 March 2021))[basins.4], of which There about were about 70 million 200 million now live people in the in theYangtze Yangtze Delta. basin The inthe population 1960s and, since then, it has increased to about 400 million (http://www.stats.gov.cn/tjsj/ndsj/ (accessed on of the Nile delta was about 43 million in the 1980s, and it increased to nearly 100 million 2 March 2021)) [4], of which about 70 million now live in the Yangtze Delta. The population by 2019, almostof all the of Nile whom delta live was in about the delta 43 million (https://databank.worldbank.org/home.aspx in the 1980s, and it increased to nearly 100 million by (accessed on 2 March2019, almost 2021) all)[5 of]. whom live in the delta (https://databank.worldbank.org/home.aspx The Yangtze(accessed River, on including 2 March 2021))its delta [5]., is located in the monsoonal climate zone of East Asia, while theThe Nile Yangtze delta is River, arid in including NW-Africa, its delta, although is located its upper in the basin monsoonal is also climate highly zone of monsoonal. Geologically,East Asia, while the Yangtze the Nile is delta composed is arid of in many NW-Africa, large- althoughscale sub its-basins upper basin is also (Figure 1A) throughouthighly monsoonal. the entire Geologically, basin, but thethe YangtzeNile has is fewer composed tributaries, of many large-scalemostly in sub-basinsthe upper basin (Figure(Figure 1B).1A ) throughoutDuring past the decades, entire basin, numerous but the Niledams has have fewer been tributaries, built in mostly the in the upper basin (Figure1B). During past decades, numerous dams have been built in the Yangtze basin, including the Three Gorges Dam (TGD, closed in 2003, and about 1800 km Yangtze basin, including the Three Gorges Dam (TGD, closed in 2003, and about 1800 km from the coast).from In the coast).Nile basin, In the there Nile basin,is only there one is major only onedam, major the dam,Aswan the High Aswan Dam High Dam (AHD) dam on(AHD) the mainstream, dam on the mainstream, which was which closed was in closed 1964 inand 1964 about and about 1000 1000 km kmfrom from the the coast. coast. UnderstandingUnderstanding the position the position of the dam of the sites dam in sites the inriver the riverbasin basin is important is important in stud- in studying ying riverine materialriverine transport material transport (discussion (discussion given givenbelow). below). Figure 1. Figure(A) The 1. Yangtze (A) The River Yangtze basin, River showing basin, the locationshowing of the the location Three Gorges of the Dam Three (TGD) Gorges and DatongDam (TGD) hydro-gauging and station. (BDatong) The Nile hydro River-gauging basin, showing station. the (B location) The Nile of the River Aswan basin, High showing Dam (AHD). the Thelocation lithology of the of bothAswan river High basins is modifiedDam from [(AHD).6,7]. The lithology of both river basins is modified from [6,7]. Water 2021, 13, 1145 3 of 10 Water 2021, 13, 1145 3 of 10 Inevitably, dam construction has changed the river water from high to low turbidity. N- andInevitably, P-fluxes dam have construction increased significantly has changed in the both river systems water, frombut Si high flux to has low decreased. turbidity. N-This and had P-fluxes led to deleterious have increased ecological significantly responses in bothin both systems, systems. but The Si fluxgoal has of this decreased. review Thiswas to had discuss led to the deleterious scientific ecologicalunderstanding responses of how in these both changes systems. have The impacted goal of this these review two wassystems to discuss in order the to scientific provide understanding insights for their of howmanagement.