key: cord-0298092-2nz3d122 authors: Scharler, UM; Bownes, SJ; Jerling, H title: How drought affects range and variability of environmental conditions in subtropical South African estuaries date: 2022-02-17 journal: bioRxiv DOI: 10.1101/2022.02.17.480947 sha: 9e1dee305ad92568228aa82b7a3a043ad97dc13f doc_id: 298092 cord_uid: 2nz3d122 Globally estuaries are under increasing pressure from human development impacts as well as the growing effects of climate change that is already, and increasingly so, causing major changes in events such as droughts. Although droughts are regular occurrences and are projected to become progressively more significant in many parts of the world, their potential impacts on estuaries requires better understanding and quantification. This study aims to quantify environmental and phytoplankton parameter changes in two contrasting subtropical estuary types in South Africa; the temporarily closed uMdloti Estuary and the predominantly open to the sea uMlalazi Estuary during a hydrological drought event and subsequent non-drought period. Drought caused lower salinities, almost exclusively freshwater, throughout the temporarily closed estuarine type uMdloti. The marine-estuarine salinity environment of the predominantly open uMlalazi during drought expanded to include lower salinities in the non-drought period. Dissolved oxygen levels were more variable during drought conditions including periods of hypoxia, but more stable at elevated levels in both estuaries during the non- drought period. Temperature measurements included higher values during drought, especially in the dry season. Chl-a concentrations were overall higher and more variable during drought in uMdloti, with periods of bloom formation as well as decay. Drought-induced conditions can span several generations for estuarine and estuarine associated organisms. The results from the study provide quantitative support for the hypothesis that extended periods of drought conditions lead to a decrease in niche availability to estuarine organisms in a range of estuary types. stenohaline estuarine taxa (Smyth and Elliott, 2016) . Temperature influences the metabolic activity and some estuarine benthic animals are actually adapted to short periods of hypoxia (Sagasti et al., 2000) . 78 Tightly coupled with changes in DO concentrations are those of pH. During autotroph bloom 79 conditions, the use of CO 2 and increase of oxygen production by primary producers such as 80 phytoplankton and macroalgae increases pH. This bloom condition is followed by decay and with temperature to influence metabolism, or predator-prey relationships through changes in prey 86 vulnerability (e.g. calcifiers) or species performance (Gaylord et al., 2015) . 87 In short, estuarine biotic communities reflect their environmental conditions, and undergo biodiversity 88 and community changes as a result of it. The natural spatial and temporal variations in estuarine 89 environments therefore support the biodiversity, productivity and functioning of estuaries (Elliott and year maintained typical spatial salinity gradients across the estuary, whereas low flow conditions 230 during drought resulted in reduced salinity variation with poly-to euhaline salinities throughout. The 231 estuary was overall cooler during the non-drought period (Fig 3) . In both estuaries, the temperature 232 environment was less variable during the non-drought period's dry season and the high temperatures 233 measured during the drought period were absent (Fig 3) . pH was predominantly above 7 during the 234 drought period, and featured lower values during the non-drought's dry season resulting in a more 235 variable pH environment especially during the dry season (Fig 2, 3 ). The variation in environmental parameters between drought and non-drought periods is partially values for both estuaries, and larger during the dry season (Fig 5) . Of the phytoplankton size fractions, 299 the microphytoplankton showed the largest divergence peaks in both estuaries. The information on 300 divergences illustrated that distinct value ranges are affected differently rather than the entire value 301 range uniformly. Differences in spatial gradients: 303 Lastly, we wanted to know whether there were also differences in estuarine axial gradients, i.e. whether 304 the habitat niches expand or contract from the upper to the lower reaches, or whether the gradients 305 remain the same? In the LTCE uMdloti, salinity gradients were the only ones to expand slightly from 306 the drought to the non-drought period, whereas temperature, pH and DO concentration gradients 307 contracted, during both wet and dry seasons (Fig 5, Table 1 ). In uMlalazi Estuary, salinity and pH 308 gradients expanded, and temperature and DO concentration gradients contracted from the drought to 309 the non-drought period (Fig 5, Table 1 ). When present, salinity, pH and DO concentration gradients Table 1 . The horizontal dotted line denotes zero divergence. The two vertical dotted lines separate the lower 345 from the middle, and the middle from the higher range values. This study provides knowledge specifically on estuarine response to an El Niño induced drought. We is high (Supplementary material, Fig A) . The relationship is however not statistically significant. DO Acknowledgements: 504 We would like to thank the following people for assistance in the field: Fru Azinwi Nche-Fambo, Shown is the time period with the most complete dataset for both phytoplankton and 746 microphytobenthos data over the study period (this study). Summary for Policymakers. The Physical Science Basis. Contribution of Working Group 608 I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Climate Change 2021 A comparative study of zooplankton dynamics in 613 two subtropical temporarily open / closed estuaries, South Africa The Effects of Altered Freshwater Inflows on Catch 616 Rates of Non-Estuarine-Dependent Fish in a Multispecies Nearshore Line-Fishery Physico-chemical and microalgal gradients change rapidly in 619 response to mouth closure in a predominantly open estuary The macrobenthos of the Mlalazi estuary South African droughts 624 and decadal variability Shifts in estuarine 626 zooplankton variability following extreme climate events: A comparison between drought and regular 627 years Closed/Open Lakes and Lagoons: Their global distribution and boundary conditions Effects 632 of Annual Droughts on Fish Communities in Mississippi Sound Estuaries Temperature, salinity and food thresholds in two brackish-water 635 bacterivorous nematode species: assessing niches from food absorption and respiration experiments Assessment of Meteorological Drought and Wet Conditions 638 Seasonal dynamics of meiofauna in a South African 641 temporarily open/closed estuary (Mdloti Estuary, Indian Ocean) How drought changes subtropical estuaries 38 The influence of mouth status on pH 644 variability in small temporarily closed estuaries Inlet mouth phase influences density, 647 variability and standing stocks of plankton assemblages in temporarily open / closed estuaries Temperature dependence of fish performance 651 in the wild: links with species biogeography and physiological thermal tolerance Influence of a Drought Event on Hydrological Characteristics of a Small Estuary on the 655 Temporarily open/closed estuaries in South Africa From synoptic to interdecadal 661 variability in southern African rainfall: Toward a unified view across time scales Quantifying rainfall seasonality across South Africa on 664 the basis of the relationship between rainfall and temperature Epifaunal communities thrive in an estuary with 667 hypoxic episodes Climate change rapidly warms and acidifies Australian 669 estuaries The influence of catchment management on salinity, nutrient 671 stochiometry and phytoplankton biomass of Eastern Cape estuaries Effects of prolonged mouth closure in 674 a temporarily open / closed estuary : a summary of the responses of invertebrate communities in the 675 uMdloti Estuary Contrasting effects of managed 678 opening regimes on water quality in two intermittently closed and open coastal lakes Stressors in the Marine Environment: Physiological How drought changes subtropical estuaries 40 682 and Ecological Responses Annual State of the Climate of South Africa LaplacesDemon: A Complete Environment for Bayesian Inference within R 687 Temperature -induced variability in metabolic 689 activity of ecologically important estuarine macrobenthos Extension of a qualitative model on nutrient 692 cycling and transformation to include microtidal estuaries on wave-dominated coasts: Southern 693 hemisphere perspective Phytoplankton biomass and size structure in two 696 South African eutrophic, temporarily open/closed estuaries Estuarine Realm. South African National Biodiversity Institute (SANBI) and CSIR An Estuary Ecosystem Classification that encompasses How drought changes subtropical estuaries 41 biogeography and a high diversity of types in support of protection and management Temperature effects on oxygen thresholds for hypoxia in 705 marine benthic organisms Shifts in small-bodied fish assemblages 708 resulting from drought-induced water level recession in terminating lakes of the Murray-Darling Basin Microtidal estuaries warrant special management 711 measures that recognise their critical vulnerability to pollution and climate change Severe droughts reduce 714 estuarine primary productivity with cascading effects on higher trophic levels An 'extreme' future for estuaries? Effects of extreme climatic 717 events on estuarine water quality and ecology The role of pioneers as indicators of biogeographic range expansion caused by global How drought changes subtropical estuaries 42 721 change in southern African coastal waters When the flathead mullet left St Lucia Estuarine Ecohydrology Climate affects fish-kill events in subtropical 728 estuaries of eastern Australia Reconciling the 731 temperature dependence of respiration across timescales and ecosystem types