Impact of Flash Flood Events on the Coastal Waters Around Madeira Island: The “Land Mass Effect”

Abstract

The Island Mass Effect has been primarily attributed to nutrient enhancement of waters surrounding oceanic islands due to physical processes, whereas the role of land runoff has seldom been considered. Land runoff can be particularly relevant in mountainous islands, highly susceptible to torrential rainfall that rapidly leads to flash floods. Madeira Island, located in the Northeast Atlantic Ocean, is historically known for its flash flood events, when steep streams transport high volumes of water and terrigenous material downstream. A 22-year analysis of satellite data revealed that a recent catastrophic flash flood (20 February 2010) was responsible for the most significant concentration of non-algal Suspended Particulate Matter (SPM) and Chlorophyll-a at the coast. In this context, our study aims to understand the impact of the February 2010 flash flood events on coastal waters, by assessing the impact of spatial and temporal variability of wind, precipitation, and river discharges. Two specific flash floods events are investigated in detail (2 and 20 February 2010), which coincided with northeasterly and southwesterly winds, respectively. Given the lack of in situ data documenting these events, a coupled air-sea-land numerical framework was used, including hydrological modeling. The dynamics of the modeled river plumes induced by flash floods were strongly influenced by the wind regimes subsequently affecting coastal circulation, which may help to explain the differences between observed SPM and Chlorophyll-a distributions. Model simulations showed that during northeasterly winds, coastal confinement of the buoyant river plume persisted on the island’s north coast, preventing offshore transport of SPM. This mechanism may have contributed to favorable conditions for phytoplankton growth, as captured by satellite-derived Chlorophyll-a in the northeastern coastal waters. On the island’s south coast, strong ocean currents generated in the eastern island flank promoted strong vertical shear, contributing to vertical mixing. During southwesterly winds, coastal confinement of the plume with strong vertical density gradient was observed on the south side. The switch to eastward winds spread the south river plume offshore, forming a filament of high Chlorophyll-a extending 70 km offshore. Our framework demonstrates a novel methodology to investigate ocean productivity around remote islands with sparse or absent field observations.

Seasonal variation in microplastics and zooplankton abundances and characteristics: The ecological vulnerability of an oceanic island system

“…1). Despite the intermittent flow of the southern rivers (Prada et al, 2005), a recent numerical study demonstrated that the streams of Madeira might play an important role in the delivery of land-based material to coastal waters during precipitation events (Rosa et al, 2022). Higher precipitation intensity recorded in April and a peak in MPs abundance in this same month (Fig.…”

“…A correlation between the monthly average precipitation and the microplastic abundance was not found in the present study, but a significant positive correlation was found between precipitation and zooplankton abundance. In comparison with estuarine or coastal environments, it can be expected that particles accumulation and dispersion in waters surrounding oceanic islands are more heavily subjected to physical oceanographic processes such as currents and winds, respect of surface run-off due to local meteorological events (Gove et al, 2016;Cardoso and Caldeira, 2021;Rosa et al, 2022). However, recent findings highlighted the underestimated impact of surface run-off episodes on suspended particulate matter and Chla concentration in waters surroundings Madeira Island (Rosa et al, 2022).…”

“…S1). Nearby is also located the river outlet of one of the largest drainage basins of the island (Ribeira dos Socorridos) (Rosa et al, 2022).…”

“…1 for station location) were provided by the Portuguese Institute for Sea and Atmosphere (IPMA). Monthly averages of satellite-derived chlorophyll-a (Chla) concentrations and daily averages of Sea Surface Temperature (SST), calculated with values within the − 2000 m isobath around the coast (as described in Rosa et al, 2022), were obtained from the Copernicus Marine Service (https://mar ine.copernicus.eu/).The satellite-derived Chla concentrations were based on a blended gridded product (Level 4; OCEAN-COLOUR_ATL_CHL_L4_REP_OBSERVATIONS_009_091) at 1 km spatial resolution (Fig. S3).…”

“…Caldeira et al, 2002;Hasegawa, 2019). However, local events of island surface run-off, with discharges of freshwater, terrigenous sediments, suspended matter and nutrients, can also significantly contribute to IME (Rosa et al, 2022). Similarly, the transport and distribution of plastic particles have been described in relation to physical processes such as ocean currents and wind (Cardoso and Caldeira, 2021;Brach et al, 2018) or precipitation events (Lima et al, 2015), and their seasonal variations can determine the convergence, accumulation, or dispersion of marine litter.…”