Influence of sediment on the growth of the invasive macrophyte Najas marina ssp. intermedia in lakes

Najas marina ssp. intermedia (Wolfg. ex Gorski) Casper (Najas intermedia) is a thermophile macrophyte species native to Central Europe, which started mass spreading across Southern Germany about 10 years ago, almost reaching the intensity of the invasive neophyte Elodea nuttallii. As part of a study to examine the spread of N. intermedia with regard to climate change, predominant populations of N. intermedia were continuously monitored. The observations revealed that some areas and lakes remained free of N. intermedia, although water temperatures and light conditions were similar to locations with predominant populations of N. intermedia. As a result, growth experiments were conducted to show that the properties of the lake sediment can affect the growth of N. intermedia.Four lakes without populations of N. intermedia and with different environmental conditions were chosen as experimental sites. During the experiments eight different sediments from four different lakes were used. Five sediments were collected from sites with extensive or predominant Najas populations, three sediments originated from locations with no or minor amounts of N. intermedia. The experiments revealed that the sediment from different lakes as well as from different locations within the same lake significantly differs in nutrient concentration and density and that those differences can affect the growth of N. intermedia. Plants growing in nutrient-rich sediment (SRP: 15 mg 100 g−1 soil, Total-P: 50 mg 100 g−1 soil) reached with 43.1 (±6.9) mm day−1, compared to 6.9 (±2.0) mm day−1 in sediment with lower nutrient concentrations (SRP: 2 mg 100 g−1 soil, Total-P: 10 mg 100 g−1 soil), higher growth rates and were less affected by the density of the sediment. The density of the sediment, on the other hand, played a significant role under conditions with low nutrient concentrations, respectively when sediments with similar nutrient concentrations were compared. For example, a comparison of sediments with a soluble phosphor concentration of 2 mg 100 g−1 soil and a total phosphor concentration of 10 mg 100 g−1 soil showed that plants growing in sediment with 75.7% particles > 0.063 mm reached twice the growth rates (6.9 ± 2.0 mm day−1) than plant growing in sediment with only 47% particles >0.063 mm (1.7 ± 0.6 mm day−1). Nutrient-poor sediment (SRP: <1 mg 100 g−1 soil, Total-P: <1 mg 100 g−1 soil), inhibited the growth of N. intermedia (0.8 ± 0.2 mm day−1). The significantly different growth rates of N. intermedia show that the lake sediment, respectively the nutrient concentration and density must be included in assessments and models regarding the growth and spread of N. intermedia.