One of the most serious problems caused by eutrophication of shallow lakes is the disappearance of submerged macrophytes and the switch to a turbid, phytoplankton-dominated state. The reduction of external nutrient loads often does not result in a change back to the macrophyte-dominated state because stabilising mechanisms that cause resilience may delay a response. Additional internal lake restoration measures may therefore be needed to decrease the concentration of total phosphorus and increase water clarity. The re-establishment of submerged macrophytes required for a long-term stability of clear water conditions, however, may still fail, or mass developments of tallgrowing species may cause nuisance for recreational use. Both cases are often not taken into account when restoration measures are planned in Germany, and existing schemes to reduce eutrophication consider the topic inadequately.Here we develop a step-by-step guideline to assess the chances of submerged macrophyte re-establishment in shallow lakes. We reviewed and rated the existing literature and case studies with special regard on (1) the impact of different internal lake restoration methods on the development of submerged macrophytes, (2) methods for the assessment of natural re-establishment, (3) requirements and methods for artificial support of submerged macrophyte development and (4) management options of macrophyte species diversity and abundance in Germany. This guideline is intended to help lake managers aiming to restore shallow lakes in Germany to critically asses and predict the potential development of submerged vegetation, taking into account the complex factors and interrelations that determine their occurrence, abundance and diversity.

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.

Non-indigenous aquatic plants are a major cause of biodiversity loss in many countries. In this study, our own field data and a literature review have been used to get an overview of the history and the present distribution of non-indigenous aquatic plant species in Germany. Results show that the number of non-indigenous aquatic plant species in Germany rose from 1 in 1860 to 12 in 1980, but doubled to 24 during the following 29 years. Thirteen of these species are naturalised in at least one federal state, 11 are only ephemeral. The number of non-indigenous aquatic plant species in the German federal states is significantly correlated with the population size and area. The increase in species number and abundance is probably caused by enhanced trading and increased invasibility of waters by eutrophication ⁄ re-oligotrophication and climate change. We propose a trading ban for some highly invasive nonindigenous aquatic species. This will not stop their natural spread, but should reduce the risk of further unintended entry and thus can be a major control factor.

Eutrophication of two urban temperate dimictic lakes in Berlin (Germany), smaller Schlachtensee (0.4 km 2 ) and larger Lake Tegel (3 km 2 ), caused total phosphorus (TP) concentrations up to 800 μg/L and a complete loss of their diverse submerged vegetation in the 1960s due to poor light conditions. Phosphorus stripping of their inflow began in the 1980s and caused a pronounced decline of their epilimnetic TP concentrations, eventually leading to reduced phytoplankton biomass and turbidity. Despite increased light availability, recovery of abundance as well as species diversity of submerged macrophytes was delayed by more than a decade, especially in the smaller lake. Slow oxidization of sapropelic sediment unsuitable for macrophyte growth, periphyton shading, herbivory, and/or lack of a viable seed bank were potential hampering factors. The present submerged vegetation, however, may already support mechanisms positively influencing water transparency such as providing habitat to enhance the ratio of piscivorous to planktivorous fish. Characeae meadows, typical for both lakes during their former mesotrophic state, so far only reoccurred in smaller Schlachtensee. Neither species composition nor abundance reversed back to the macrophyte community present in the nineteenth century. Although TP concentrations may decline further and some rare species have been detected, reassembly of this plant community will most probably not occur because many submerged macrophyte species have become rare throughout northwest Europe.

a b s t r a c tIn the present study, we present a synopsis of two macrophyte surveys of physiographic units in northwest Germany carried out over one decade. Data were used to test a set of hypotheses on macrophyte distribution at the regional level. Rank-frequency curves resembled the broken stick model. Twentyone species of the 59 most frequent species occurred at high frequencies above 15%. Helophytes made up a high percentage (12 of 21) of the frequent species. Phalaris arundinacea was the most frequent species in both sampling periods. Most species showed no considerable change in frequency over time, among them the core hydrophytes. Spatial variation of species frequencies among physiographical units showed a unimodal distribution in relation to frequency. Spatial variation of frequencies of functional groups was significantly lower. Most uneven distribution among physiographical units was found in cryptogams. DCA ordinations of physiographical units showed a spatial gradient from alluvial plains to higher grounds units, which remained constant over time. CCA ordination of physiographical units in relation to environmental parameters identified two main axes, an altitudinal gradient and an alkalinity gradient. Species composition of units corresponded to the main landscape pattern of alluvial plains, glacial lowlands, and higher grounds on Mesozoic rock. Species diversity showed a complex behavior. Diverse units were found both in alluvial plains and glacial lowlands of intermediate elevation. The study may help defining regionally differentiated reference states for stream management, benchmarking indicator scores of species and avoiding application of assessment methods outside their range of applicability.

Lake Tegel is an extreme case of restoration: inflow treatment reduced its main external phosphorus (TP) load 40-fold, sharply focused in time, and low-P water flushed the lake volume ≈ 4 times per year. We analysed 35 years of data for the time TP concentrations took to decline from ≈ 700 to 20–30 µg/l, biota to respond and cyanobacteria to become negligible. The internal load proved of minor relevance. After 10 years, TP reached 35–40 µg/l, phytoplankton biomass abruptly declined by 50% and cyanobacteria no longer dominated; yet 10 years later at TP < 20–30 µg/l they were below quantifiable levels. 20–25 years after load reduction, the lake was stably mesotrophic, macrophytes had returned down to 6–8 m, and vivianite now forms, binding P insolubly in the sediment. Bottom-up control of phytoplankton through TP proved decisive. Five intermittent years with a higher external P load caused some ‘re-eutrophication’, delaying recovery by 5 years. While some restoration responses required undercutting thresholds, particularly that of phytoplankton biomass to TP, resilience and hysteresis proved irrelevant. Future research needs to focus on the littoral zone, and for predicting time spans for recovery more generally, meta-analyses should address P load reduction in combination with flushing rates.

Nuttall’s waterweed ( Elodea nuttallii ) is the most abundant invasive aquatic plant species in several European countries. Elodea populations often follow a boom-bust cycle, but the causes and consequences of this dynamics are yet unknown. We hypothesize that both boom and bust periods can be affected by dreissenid mussel invasions. While mutual facilitations between these invaders could explain their rapid parallel expansion, subsequent competition for space might occur. To test this hypothesis, we use data on temporal changes in the water quality and the abundance of E. nuttallii and the quagga mussel Dreissena rostriformis bugensis in a temperate shallow lake. Lake Müggelsee (Germany) was turbid and devoid of submerged macrophytes for 20 years (1970–1989), but re-colonization with macrophytes started in 1990 upon reductions in nutrient loading. We mapped macrophyte abundance from 1999 and mussel abundance from 2011 onwards. E. nuttallii was first detected in 2011, spread rapidly, and was the most abundant macrophyte species by 2017. Native macrophyte species were not replaced, but spread more slowly, resulting in an overall increase in macrophyte coverage to 25% of the lake surface. The increased abundance of E. nuttallii was paralleled by increasing water clarity and decreasing total phosphorus concentrations in the water. These changes were attributed to a rapid invasion by quagga mussels in 2012. In 2017, they covered about one-third of the lake area, with mean abundances of 3,600 mussels m −2 , filtering up to twice the lake’s volume every day. The increasing light availability in deeper littoral areas supported the rapid spread of waterweed, while in turn waterweed provided surface for mussel colonization. Quantities of dreissenid mussels and E. nuttallii measured at 24 locations were significantly correlated in 2016, and yearly means of E. nuttallii quantities increased with increasing mean dreissenid mussel quantities between 2011 and 2018. In 2018, both E. nuttallii and dreissenid abundances declined. These data imply that invasive waterweed and quagga mussels initially facilitated their establishment, supporting the invasional meltdown hypothesis, while subsequently competition for space may have occurred. Such temporal changes in invasive species interaction might contribute to the boom-bust dynamics that have been observed in Elodea populations.

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