Evaluation of Macrophyte Community Dynamics (2015–2020) in Southern Lake Garda (Italy) from Sentinel-2 Data

Nicola Ghirardi,Claudia Giardino,Mariano Bresciani,Monica Pinardi,Gary Free,Rossano Bolpagni

doi:10.3390/app12052693

Abstract

Macrophytes are of fundamental importance to the functioning of lake ecosystems. They provide structure, habitat, and a food source and are a required component in monitoring programs of lake ecological quality. The key aim of this study is to document the variation in spatial extent and density of macrophytes seasonally between 2015 and 2020 of the Sirmione Peninsula (Lake Garda, Italy), using Sentinel-2 imagery. In addition to this, our results were compared to previous data from imaging spectrometry; individual parameters affecting macrophyte communities were tested, and the possible effect of the COVID-19 lockdown on macrophyte colonization was evaluated. Satellite images allowed the mapping of the spatiotemporal dynamics of submerged rooted macrophytes in order to support monitoring of the shallow water ecosystem under study. Substantial changes were found in both spatial extent and density over the period from 2015 to 2020, particularly in 2019 when there was almost a complete absence of dense macrophytes. Variables found to influence the amount of macrophytes included transparency, chlorophyll–a, water level, winter wave height, and grazing by herbivores. A separate analysis focusing on areas associated with boat transit found a recovery in macrophyte coverage during the period of COVID-19 lockdown. The outcome of the study highlights a decline in the density of the macrophytes and a shift towards deeper areas compared to the situation in 1997. The area examined is part of an internationally important site containing the highest abundance and diversity of overwintering water birds in Italy. Exploiting satellite data at high frequency provided an insight to understand the dynamic changes and interactions with herbivorous birds, environmental factors, and anthropogenic pressures, revealing a delicately balanced and threatened ecosystem.

Highlights

This article is an open access articleThe issue of the dynamics of primary producers of lake ecosystems is acquiring more and more relevance because of increasingly marked effects of climate change on the functioning of these systems [1,2]
The results of the comparison of the spectral signatures as produced by the 6SV for the three invariant surfaces attested to the quality of the atmospheric correction: for all the S2 images the coefficient of variation is low: 0.26 for sandy beach, 0.23 for evergreen vegetation, and 0.14 for rooftops
By adding the previous validating studies with S2 over Lake Garda [54,55] we might conclude that the atmospheric correction carried out by 6SV is robust and coherent in order to obtain temporal remote sensing reflectances (Rrs) that are comparable

Summary

Introduction

This article is an open access articleThe issue of the dynamics of primary producers of lake ecosystems is acquiring more and more relevance because of increasingly marked effects of climate change on the functioning of these systems [1,2]. An unexpected dynamism was observed, suggesting the existence of rapid cycles (e.g., of the order of a few years) characterized by the ability of macrophytes—both vascular and algae of the Charophyceae class—to expand or contract colonized areas along the depth gradient by a few meters [7,8]. These growing depth variations— of limited magnitude—should be evaluated as very significant, considering that the depth range colonized by macrophytes hardly exceeds 15–20 m in deep oligo–mesotrophic temperate lakes [11]. In large lakes, deviations of a few decimeters of the maximum growing depth of macrophytes can result in large variations of colonized areas (of the order of hectares of surface) [4]

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