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Socio-ecological change in the Ruvu Estuary in Tanzania, inferred from land-use and land-cover (LULC) analysis and estuarine fisheries

Ecosystem goods and services derived from estuaries have sustained coastal livelihoods in the Western Indian Ocean (WIO) region throughout recorded history. Estuaries provide fertile and seasonally irrigated space for planting crops, mangrove products for construction and fuel, and fish as a protein source. Human population growth and an escalating demand for natural resources threaten estuarine critical habitats and their functioning, exacerbated by the effects of climate change. Decadal and seasonal land-use and land-cover (LULC) changes in the Ruvu Estuary in Tanzania were investigated through analysis of Landsat 5/8 and Sentinel-2 satellite images. The estuary is river-dominated and truncated near the coast during high river flow, with tidal influence extending approximately 12 km upstream during low river flow. LULC change detection targeting nine classes (water, developed, barren, forest, grasslands, cultivated, mangroves, wetlands and mudflats) showed that estuary-associated wetlands and mangroves had declined significantly over the past two decades (1995-2016) making way for developed land (growth of Bagamoyo Town), cultivated land (agricultural expansion with increasing population) and grasslands (coastal habitat changes). Seasonal LULC changes were conversion of wetlands to cultivated land after the wet season, and transformation of fallow wetlands to grasslands. The estuarine fishery relied on a small number of mainly freshwater and marine migrant species, compared to a highly diverse mix of mainly marine species in the nearby coastal fishery. The sparsity of quantitative fisheries data, spectral confusion when modelling land-cover change, and absence of household survey data to assess livelihood activities remain major information gaps. Generalized recommendations for improving socio-ecological change studies in WIO estuarine systems are provided.


Introduction
Estuaries are amongst the most productive ecosystems in the world (Costanza et al., 1997) and have supported a range of rural to urban coastal communities throughout recorded history (Gari et al., 2015). In the Western Indian Ocean (WIO), over-exploitation by fast-growing human populations now threatens the ecological functioning of estuaries and the essential benefits derived from them (Hamerlynck et al., 2010;Barbier et al., 2011;papers in Diop et al., 2016).
For example, reduced freshwater inflow because of damming or freshwater extraction in upstream catchment areas threaten critical estuarine habitats (e.g., mangroves; Friess et al., 2019) which in turn affects estuarine nursery function (Gillanders and Kingsford, 2002) and therefore recruitment of juvenile fish and prawns to fished populations. Local over-exploitation of goods and services in estuaries, such as harvesting of mangroves, fishing and encroachment worsen the effects of freshwater scarcity, hastening the degradation of critical habitats (Diop et al., 2016).

Abstract
Ecosystem goods and services derived from estuaries have sustained coastal livelihoods in the Western Indian Ocean (WIO) region throughout recorded history. Estuaries provide fertile and seasonally irrigated space for planting crops, mangrove products for construction and fuel, and fish as a protein source. Human population growth and an escalating demand for natural resources threaten estuarine critical habitats and their functioning, exacerbated by the effects of climate change. Decadal and seasonal land-use and land-cover (LULC) changes in the Ruvu Estuary in Tanzania were investigated through analysis of Landsat 5/8 and Sentinel-2 satellite images. The estuary is river-dominated and truncated near the coast during high river flow, with tidal influence extending approximately 12 km upstream during low river flow. LULC change detection targeting nine classes (water, developed, barren, forest, grasslands, cultivated, mangroves, wetlands and mudflats) showed that estuary-associated wetlands and mangroves had declined significantly over the past two decades  making way for developed land (growth of Bagamoyo Town), cultivated land (agricultural expansion with increasing population) and grasslands (coastal habitat changes). Seasonal LULC changes were conversion of wetlands to cultivated land after the wet season, and transformation of fallow wetlands to grasslands. The estuarine fishery relied on a small number of mainly freshwater and marine migrant species, compared to a highly diverse mix of mainly marine species in the nearby coastal fishery.
The sparsity of quantitative fisheries data, spectral confusion when modelling land-cover change, and absence of household survey data to assess livelihood activities remain major information gaps. Generalized recommendations for improving socio-ecological change studies in WIO estuarine systems are provided.
Keywords: Estuarize-WIO, Ruvu Estuary, land-use/ land-cover analysis, small-scale fisheries Socio-ecological change in the Ruvu Estuary in Tanzania, inferred from land-use and land-cover (LULC) analysis and estuarine fisheries Socio-ecological systems (SES) assessments of estuaries can be complex with a high data demand -yet the interactions between the human and natural systems cannot be ignored if estuaries are to continue to provide natural resources for livelihoods (Milner-Gulland, 2012). As a case study in a data-poor environment, a SES assessment of the Ruvu Estuary in Tanzania was undertaken. The area has been settled for millennia by a succession of civilizations (Mosha and Plevoets, 2020) and is a highly productive part of the coast that supplies local and distant markets with fish and agricultural products (Mkama et al., 2010). SES at the Ruvu Estuary, as elsewhere in the WIO, are dominated by fish-based farming (FBF) systems (Hamerlynck et al., 2020;Francisco et al., 2021;Furaca et al., 2021;Mwamlavya et al., 2021), in which households derive some 30 to 50 % of their income from fisheries and engage in a wide livelihood portfolio, including farming, use of mangrove products, livestock herding, bee keeping and operating small business enterprises (Hamerlynck et al., 2020).
Seasonal and decadal changes in land-use and landcover (LULC) are good indicators of socio-ecological interactions, because they provide information on hydrological and ecological conditions that govern natural capital use (e.g., Ngondo et al., 2021;Taylor and Suthers, 2021). The aims of this study were to: assess seasonal and decadal change in LULC based on an analysis of satellite images downloaded from NASA's Landsat 5/8 and ESA's Sentinel-2 programmes; and infer the importance of estuarine fisheries in FBF systems at the Ruvu Estuary based on observational and published information.
A preliminary socio-ecological change assessment of the Ruvu Estuary over the past two decades and seasonally is provided, with recommendations for future research in data-poor estuarine systems of the tropical WIO.

Study area
The Ruvu Estuary receives freshwater inflow from the Wami Ruvu Basin (WRB) and discharges into the WIO on the mainland side of the Zanzibar Channel, some 5 km north of the coastal town of Bagamoyo   and direct freshwater flow into the estuary reaches a monthly maximum of 150 m 3 /s at Morogoro Bridge, some 45 km upstream of the estuary mouth (GLOWS-FIU, 2014b;2014c). Upstream of the estuary, the Ruvu River flows through major agricultural, industrial, and urban areas where freshwater is diverted for irrigation, industrial, aquaculture and domestic use (Ngondo et al., 2021). With rapid coastal expansion, particularly at Dar es Salaam where there is a critical water supply/ demand deficit, the WRB is of high strategic importance and has been the subject of numerous impact assessments, predictive models, policy documents and published research (e.g., GLOWS-FIU, 2014ad;van Eeden et al., 2016;Mdee, 2017;Alphayo and Sharma, 2018;Miraji et al., 2019;Ngondo et al., 2021). The area of interest (AOI) for this study was taken as the final 10 to 12 km of the Ruvu Estuary, to include the lower river, the river-estuary transition, estuary (based on upstream salinity penetration; GLOWS-FIU, 2014d), and an area of the offshore bay highly influenced by river/estuarine water during tidal exchange and floods. Eight zones within the AOI were identified according to proximity to the estuary: estuary-supporting habitats (zones 1-2); river adjacent and river influencing landscape (zone 3); urbanized land including Bagamoyo Town (zones 4 and 5); and land influenced by this urbanization to the south. Bagamoyo Town lies outside the estuarine functional zone but was included in the LULC assessment because its urban growth (built-up area and population size) was considered a major driver of socio-ecological change and anthropogenic impact on estuarine resources (summarized in Groeneveld et al., 2021). Zones 6-7 are less urban-influenced and span some of the land between the Ruvu and the Wami estuaries to the north, and zone 8 includes the offshore plume area off the Ruvu mouth. The study area comprised 458 km 2 , of which 358 km 2 was land.

Catchment basin
Information on rainfall, river discharge, water level, geology, hydrogeology, administration, infrastructure and population demographics in the catchment area of the Ruvu River were obtained from an online 'Digital Atlas of Water Resources' (http://glows/fiu. edu). Geographically, the information originates from outside the estuarine study area, but is crucial for quantifying the variability of long term and seasonal freshwater discharge into the estuary. Spatial patterns of water supply, demand and use at the WRB hydrological station at Morogoro Road Bridge showed peak discharge in April to May with a secondary peak in November to January (GLOWS-FIU, 2014b; http:// glows/fiu.edu). Multiple studies have shown a longer term decrease in flows to the coast (GLOWS-FIU, 2013;2014a-d) and the possible impacts across ecosystems and people (Semesi et al., 1998;Kiwango et al., 2015;Shaghude, 2016;Duvail et al., 2017;Miraji et al., 2019;Macharia et al., 2020). The lack of environmental flow management and enforcement in the WRB influences freshwater flow into the Ruvu Estuary, as a vital component of estuarine ecology (e.g., nutrient processing, sediment trapping, maintenance of critical habitats; Kiwango et al., 2015) all of which are necessary to sustain livelihoods of coastal communities.

Estuary zones based on salinity profiles
Salinity is a critical variable in estuaries, which governs the distribution of biota through mixing of freshwater inflow and tidal influence at various spatio-temporal scales. Salinity gradients influence the distribution of estuarine habitats, availability of freshwater, land-cover (LC), land-use (LU) and fished resources. Defining salinity zones that correlate with biological tolerance and distribution of species throughout the estuary (some being directly targeted for subsistence by nearby communities) was the initial step. A broad-category system was adopted with oligohaline (0.5-5.0 psu), mesohaline (  Supervised classification (decadal change) and objectbased imagery analysis approaches (seasonal change) were adopted using a support vector machine classifier on a GIS (ArcMap™ GIS) and the RGB and NIR bands (Red, Green, Blue and Near-Infrared). Classification of land-cover categories used the maximum likelihood classification algorithm.

Results and Discussion
Decadal land cover change based on Landsat 5/8 imagery (1995 -2016) Landsat imagery showed a significant decline in estuary-associated wetlands from 42 % of the AOI in 1995 to 17 % in 2016 (Fig. 3). The similarity of water area coverage across all temporal comparisons (20 -23 % of the FIGURE 3 -Historical changes (interdecadal) in LC around the Ruvu Estuary. Percent cover across nine categories related to estuary, estuary supporting habitats and developed land. AOI) confirmed that studied periods had similar hydrological characteristics (Fig. 3). Mangrove-cover declined from 8 % to 5 % of the AOI in contrast to other LC classes.
Cultivated land (7 % to 14 %) and forested land ( Town over the past 21 years (Fig. 4). The spectral confusion amongst some classes (interchangeable cultivated-and wetland spectral signatures next to the estuary and mangroves and forests having similar  The analysis supported a broad seasonal pattern, with wetlands being converted to cultivated areas during the dry season, and uncultivated wetlands becoming grasslands (Fig. 6). However, the estimates of seasonal change in land cover were affected by mosaics of pixels that could alternatively be attributed to wetlands, grasslands or mudflats, caused by classification confusion from similar spectral signatures. Cultivated land next to the estuary differed substantially from cultivated land further away (e.g., between developed land parcels), attributed to differing agricultural activities.
The model did not satisfactorily distinguish between mudflats, non-vegetated wetlands, grasslands and cultivated land during the dry season. Distinguishing between cultivated land, wetlands, forests and mangroves was also challenging during the wet season. Remaining land classes had reliability of 60 -70 %.
Linear modelling of spatio-temporal changes incorporating seasonality across all AOI zones showed that zone 1 (estuary) is a unique land-cover functional area dominated by mangroves (Fig. 7). Mangroves are an important resource (wood) and ecological support service (nursery area) (Semesi et al., 1998) Table 3. There are critical declines in wetlands in all zones since the 1990s, with increasing cultivation in most zones (except zones 1 and 7).
These overall findings agree with a larger LULC study of the WRB (Ngondo et al., 2021) and are related to water resource implications. Since 1990, coastal populations have increased significantly with related demands for agricultural products and water, resulting in the conversion of wetlands to cultivated lands.
Significant changes to freshwater flow was also recognized in response to potable and agri-sector demands.  Flows are set to further decline with changes in climate, especially at the coast where rainfall has decreased since 1990 (Ngondo et al., 2021).     (Mugabe et al., 2021;Manyenze et al., 2021).

Fish and shellfish resources
Based on the available data, the estuarine fishery in the Ruvu relies on few species and is not as important to local communities as the nearshore coastal fishery,

Conclusion and recommendations
The Ruvu Estuary is strongly seasonal and river-dom-  Estuarine extent is a first step towards determining the freshwater needs of estuaries, which have been mostly overlooked in the WIO region (Kiwango et al., 2015). To improve studies of estuaries and their associated socio-ecological systems, it is recommended that: (1) short periods of continuous monitoring of estuarine flow is undertaken, to obtain the full range of physico-chemical conditions across seasons; (2) a LULC classification system suited to complex tropical socio-ecological systems is developed, to reduce spectral confusion and increase modelling accuracy; (3) random sampling of fish landings in estuaries are undertaken to determine spatio-temporal variability; and (4) that household surveys of livelihood dependence on estuarine goods and services are required for constructing fully integrated socio-ecological change assessments.