Pole and pontoon hulas: An effective way of ecological engineering to increase productivity and biodiversity in the hard-substrate environment of the port of Rotterdam

Underwater environments in ports are designed for harbour activities solely. However, by simple and cost-effective measures, suitable habitat for underwater flora and fauna can be created. This is expected to have positive effects on higher trophic levels, such as fish, and improve water quality, by enlarging filter feeder biomass. In this study we developed 'pole hulas' and 'pontoon hulas', consisting of hanging ropes of different materials. The pole hulas are made up of many 6. mm thick and 55. cm long strings just above and below the mean low water level (MLWL) around poles. The pontoon hulas resemble raft like structures with 12. mm thick and 150. cm long ropes within the open space of mooring pontoons. The first experimentation with these structures was executed in the polyhaline harbours of the port of Rotterdam. The pole hulas were rapidly colonised by a variety of organisms. Above MLWL a seaweed community dominated on the strings. Below MLWL Mytilus edulis (the Blue mussel) was found to be the dominating species after a few months. In the dense layer of M. edulis on both pole hulas and pontoon hulas many mobile soft-bottom amphipods and young ragworms occurred, which means that colonisation on these structures compensate for biodiversity loss of bottom fauna due to dredging and disturbance by propellers of ships. Settlement of the exotic Crassostrea gigas (Pacific or Japanese oyster) did not occur on the strings of the pole hulas, the ropes of the pontoon hulas and not on the poles with hulas.Wet biomass (including shells) on pole hulas was positively correlated with depth and on average 4.4-11.4 times higher compared to biomass on reference poles. Colonization of the pontoon hula ropes was similar to colonization of the pole hulas below MLWL. Biomass production per rope was density dependent and optimal density of ropes was estimated at 4-8ropesm ^-2. Biomass (mainly M. edulis) on the ropes of pontoon hulas decreased to a half from the edge to the heart of the hula demonstrating the limitation of food by competition.It was concluded that ecological engineering in the port of Rotterdam with simple structures such as pole and pontoon hulas strongly enhances sessile biological production and biodiversity. This is likely to result in a positive impact on local water quality, and, if applied at larger scales, may have positive influences on the remains of Rhine-Meuse estuary.