The background for this study was NIVA's observations and warning in May 2013, about the absence of eelgrass in a previously mapped, nationally important eelgrass bed in Slependrenna, which extends across the boundaries of Bærum and Asker municipality. On behalf of the County Governor of Oslo and Akershus, we have done surveys to follow-up the distribution and condition of this eelgrass bed. Field surveys were conducted in 2017, 2018 and 2019, and showed that the earlier mapped eelgrass bed was not permanently lost, and that its distribution today (September 2019) is approximately the same as in 2008. In 2018, algae material was collected for DNA analysis and species identification of a possible invasive red algae; Gracilaria vermiculophylla. In September 2019, additional sampling was carried out for a semi-quantitative assessment of the amount ratio between G. vermiculophylla and the native species G. gracilis. The surveys showed high density of eelgrass in the meadow in each of the years examined. However, the eelgrass bed was occasionally strongly affected by turf algae, which is a known threat to eelgrass beds, and one of the factors that has caused a loss of 60 % of the eelgrass beds on the Swedish coast. The DNA analyzes revealed presence of the invasive red alga G. vermiculophylla. In addition to the high prevalence of turf algae, a relatively high prevalence and occurrence of the invasive red algae was observed. Spread of this red alga poses a high ecological risk to sheltered ecosystems such as those found in Slependrenna. G. vermiculophylla was mostly found at the deepest parts of Slependrenna (around 5 meters depth), which is around the maximum lower growth limit for eelgrass in the inner Oslofjord, and this indicates that the species does not yet have a major negative impact on eelgrass occurrence. The spread potential and the high ecological risk of the species indicate that the distribution of the species should be monitored. The threats associated with eutrophication, turf algae, and black-listed species with high ecological risk come on top of other well-known threats such as physical development, establishment of marinas and artificial sandy beaches, global warming and acidification. The sum of these factors makes the threat picture for the eelgrass bed in the area very high.