Response of Submerged Macrophyte Communities to External and Internal Restoration Measures in North Temperate Shallow Lakes.

Sabine Hilt,Irmgard Blindow,Thomas A Davidson,Ellen Van Donk,Mikael Gillefalk,Geoff Phillips,Carl D Sayer,Wolf M Mooij,Marta M Alirangues Nuñez,Jan Köhler,Nigel Willby,Elisabeth S Bakker,Erik Jeppesen,Jacqueline Rücker,Klaus Van De Weyer,Ruurd Noordhuis,Torben L Lauridsen,Jan H Janse,Sven Teurlincx,Andrea Kelly,Arno Waterstraat,Timm Kabus,Martin Søndergaard,Hans Heinrich Schuster ,Lars Hansson ,Annette B.g Janssen

doi:10.3389/fpls.2018.00194

Abstract

Submerged macrophytes play a key role in north temperate shallow lakes by stabilizing clear-water conditions. Eutrophication has resulted in macrophyte loss and shifts to turbid conditions in many lakes. Considerable efforts have been devoted to shallow lake restoration in many countries, but long-term success depends on a stable recovery of submerged macrophytes. However, recovery patterns vary widely and remain to be fully understood. We hypothesize that reduced external nutrient loading leads to an intermediate recovery state with clear spring and turbid summer conditions similar to the pattern described for eutrophication. In contrast, lake internal restoration measures can result in transient clear-water conditions both in spring and summer and reversals to turbid conditions. Furthermore, we hypothesize that these contrasting restoration measures result in different macrophyte species composition, with added implications for seasonal dynamics due to differences in plant traits. To test these hypotheses, we analyzed data on water quality and submerged macrophytes from 49 north temperate shallow lakes that were in a turbid state and subjected to restoration measures. To study the dynamics of macrophytes during nutrient load reduction, we adapted the ecosystem model PCLake. Our survey and model simulations revealed the existence of an intermediate recovery state upon reduced external nutrient loading, characterized by spring clear-water phases and turbid summers, whereas internal lake restoration measures often resulted in clear-water conditions in spring and summer with returns to turbid conditions after some years. External and internal lake restoration measures resulted in different macrophyte communities. The intermediate recovery state following reduced nutrient loading is characterized by a few macrophyte species (mainly pondweeds) that can resist wave action allowing survival in shallow areas, germinate early in spring, have energy-rich vegetative propagules facilitating rapid initial growth and that can complete their life cycle by early summer. Later in the growing season these plants are, according to our simulations, outcompeted by periphyton, leading to late-summer phytoplankton blooms. Internal lake restoration measures often coincide with a rapid but transient colonization by hornworts, waterweeds or charophytes. Stable clear-water conditions and a diverse macrophyte flora only occurred decades after external nutrient load reduction or when measures were combined.

Highlights

Shallow lakes are the most abundant freshwater ecosystems on earth (Verpoorter et al, 2014)
Our literature review provided information on water quality and macrophyte development in 21 turbid lakes that were subject to external nutrient loading reduction without additional in-lake measures (Table 1) and 28 lakes with inlake restorative measures
Our analyses suggest that the composition of the macrophyte community and their seasonal abundance in shallow lakes during recovery from turbid, highly eutrophic conditions often depends on remnant macrophyte stands, the specific restoration measure applied and additional stochastic influences on water clarity such as winter fish kills, cormorant predation on fish or introduction of invasive filter-feeding mussel populations

Summary

Introduction

Shallow lakes are the most abundant freshwater ecosystems on earth (Verpoorter et al, 2014). Sayer et al (2010a) suggested a typical pattern of lake macrophyte loss, defining a so-called “crashing” state lying between the stable clear-water state featuring a diverse plant community and the final turbid state lacking in macrophytes. This crashing state is characterized by the occurrence of only a few macrophyte species that can complete their life cycle during clear-water conditions in spring and early summer while later in summer, cyanobacteria blooms often occur. The remaining macrophyte stands are lost and give way to yearround phytoplankton dominance (Sayer et al, 2010a,b, 2016) Under these conditions, several ecosystem functions and services deteriorate, including biodiversity support, nutrient retention, provision of water of drinking or swimming quality (Hilt et al, 2017)

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