The invasive red alga Gracilaria vermiculophylla in the Baltic Sea: adaptation to brackish water may compensate for light limitation

The recent introduction of Gracilaria vermiculophylla (Rhodophyta) to the Kiel Fjord area was a reason for concern, since this red macroalga perfoms best under mesohaline conditions and thus appears well adapted to thrive and spread in the Baltic Sea environment. A systematic survey on a coastal range of 500 km in 2006 and 2007 indicated considerable multiplication and spreading of G. vermiculophylla within Kiel Fjord, but provided little evidence of long-distance transport. Nonetheless, flow-through growth experiments conducted at a range of salinities under ambient light showed that G. vermiculophylla should be able to grow in most of the Baltic Sea. Growth declined only below a salinity of 5.5. High water temperatures in summer seem to reduce resistance against low salinity. Growth of G. vermiculophylla in the SW Baltic is limited by light and is only possible during summer and above a depth of 3 m. Drifting fragments are dispersed by currents. Either they sink to deeper waters, where they degrade, or they accumulate in shallow and sheltered waters, where they form perennial mats. These overgrow not only soft bottom sediments, but also stones, which are an important habitat to Fucus vesiculosus, the main native perennial alga in the Baltic Sea. As compared to F. vesiculosus, G. vermiculophylla seems to represent a preferred refuge for mesograzers and other invertebrates, particularly in winter. Nonetheless, feeding trials showed that potential grazers avoided G. vermiculophylla relative to F. vesiculosus. Daily biomass uptake by grazers associated with G. vermiculophylla in nature did not exceed 2 g kg–1 and is <11% of average daily net growth (18.5 g kg–1) in the first 2 m below sea level. Consequently, feeding may not be sufficient to control the spread of G. vermiculophylla in the SW Baltic. Our study suggests that absence of feeding enemies and adaptation to brackish water may allow G. vermiculophylla to invade most shallow coastal waters of the inner Baltic Sea despite light limitation.