State of the Environment

In a series of documents we wish to present news and views on the state of the environment, more precisely the various basins of the Dutch Delta. Do we see changes in the (benthic) communities of these basins as we sample them through time? Do we see similar trends for the various basins or do they behave independently? What are the inferences of these observations and what does it tell us about the quality of the environment?

Invaders in the Dutch delta : The arrival, development and impact of exotic macrofauna species in the Dutch delta waters.

by Sander Wijnhoven, Pim van Avesaath & Herman Hummel

(in cooperation with Rijkswaterstaat of the Ministry of Infrastructure and Environment, and the Team Invasive Species of the Ministry of Economic Affairs, Agriculture and Innovation)

Fifty years of monitoring data on the macrofauna communities of the soft sediments in the Dutch Delta waters showed specific patterns of arrival, development and impact of exotic species.



Figure 1. Comparing the number of exotic species per water body and the ways of introduction of exotic species to the Dutch delta in time. The ways of introduction and the year of appearance for species that arrived earlier than the 1960s (when our observations started, yet were then present already) have been based on literature (indicated in Wijnhoven & Hummel 2009)

Till 2008 a total of 25 exotic species was observed. Differences in numbers of exotic species between water bodies (Fig. 1) may have to do with differences in the characteristics of the systems, like the water quality or the degree of “openness” (open estuary or enclosed basin), or differences in the use of the system, like the presence of aquaculture causing higher numbers of introductions. Of the 25 observed exotic species, 21 settled successfully which means that they are found every year in considerable numbers indicating that they can survive and reproduce in our waters. Several species are so successful that they completely dominate parts of the Dutch delta systems. When such successful exotic species have a large impact on their surroundings (e.g. change their environment or outcompete other species), and cause large economic damage or are a risk for human health, these species are called invaders. On soft sediments, 6 species can be called invasive, especially as there is a lot of money concerned in negating their negative impacts, or as they completely change the nature of their environment.

Table 1. Overview of the invasive species present in the soft sediment communities of the Dutch delta.

Slipper limpet (Crepidula fornicata) Source: www.marinespecies.org Photo: F. Nuyttens	Introduced with oysters from North America (in the delta since 1926).	Overgrows shellfish culture plots, competes with suspension feeders like most bivalves for food and space; completely dominates the communities of Lake Grevelingen.

American Jack knife (Ensis directus) Source: www.marinespecies.org Photo: F. Nuyttens	Introduced via ballast water coming from North America (in the delta since 1988).	Competes with suspension feeders like most bivalves for food and space; very abundant in the Oosterschelde and the western part of the Westerschelde.

Softshell clam (Mya arenaria) Source: www.marinespecies.org Photo: M. Decleer	Probably introduced via shipping hull from North America by the Vikings in the 13^th century.	Competes with suspension feeders like most bivalves for food and space; completely dominates the communities of Lake Veerse Meer.

Pacific oyster (Crassostrea gigas) Source: www.aquatic-aliens.de Photo: S. Nehring	Actively introduced for aquaculture from East Asia in 1964.	Competes with suspension feeders like most bivalves for food and space and turns soft sediments into hard substrates; very abundant in the Oosterschelde

Brush-clawed shore crab (Hemigrapsus takanoi) Source: www.marinespecies.org Photo: M. Decleer	Initially introduced in Europe via ballast water from the North-west Pacfic, introduced via oyster transport into the delta (1999).	Seems to replace the Common shore crab (Carcinus meanas) in the Oosterschelde and the Westerschelde.

Australian tubeworm (Ficopomatus enigmaticus) Source: www.europe-aliens.org Photo: D. Minchin	Probably introduced as shipping hull or via ballast water from the South-west Pacific (in the delta since 1967).	Overgrows hard substrates and structures; as it is a brackish water species, problems in the delta are currently of local nature.

Nowadays a substantial part of our soft sediment communities appears to be of exotic origin. At present exotic species on average make up 9, 14, 17 and 23 % of the total macrofauna densities in respectively the Oosterschelde,LakeGrevelingen, the Westerschelde and Lake Veerse Meer. As several invaders are relatively heavy weighting bivalves, they make up on average 45, 74, 40 and 81 % of the total macrofauna biomass, respectively.


Fig. 2. Developments in exotic densities and biomass (Bivalvia = clams, Gastropoda = snails, Polychaeta = bristle worms, Others = other macrofauna) compared to densities and biomass of the native macrofauna for Lake Veerse Meer during the period 1990-2008.

Bristle worms are generally the most abundant group of macrofauna which is also the case concerning the exotic macrofauna (Fig. 2). In biomass the clams are often the dominant group for both native and exotic macrofauna (Fig. 2). The example of Lake Veerse Meer shows a sharp decrease in biomass from 2001 to 2004 which co-occurs with severely deteriorating conditions including algal blooms and anoxic events. In 2004 the water exchange between Lake Veerse Meer and the tidal marine Oosterschelde was restored. As a result the former brackish Lake Veerse Meer became marine and the water quality (e.g. visibility, nutrients levels and oxygen conditions) improved. As a result the total macrofauna biomass increased, but also the exotic bivalves profited from the improved conditions (Fig. 2). The drop in the total numbers of macrofauna (including the exotic bristle worms) is actually a good sign as it is a well known phenomenon that at good water quality the more numerous polychaete dominated communities will turn into the less numerous bivalve dominated communities.

Fig. 3. Developments in total biomass for the American Jack knife clam after its arrival in Lake Grevelingen and the Oosterschelde.

A typical pattern in the population dynamics of exotic species can be observed after their arrival, as exemplified by the American Jack knife clam after its arrival in Lake Grevelingen in 1990 and in the Oosterschelde in 1988 (Fig. 3). An initial phase (lag-phase) with little expansion of the populations typically lasting about 10 years for soft sediment species in the Dutch delta is followed by a short period of exponential increase in the population size (expansion phase). Subsequently, densities and biomass decrease and population size starts to fluctuate similarly to that of native species. This pattern is observed for several exotic species in the various waters. Exotic species present in the waters for more than a few decades do generally show population dynamics not different from the native species.

Complete replacement of native species by exotic species is not known for the Delta, although the nowadays replacement of the Common shore crab (Carcinus maenas) by the Brush-clawed shore crab (Hemigrapsus takanoi) seems to be a serious threat.

It may be concluded from this study that the population dynamics of settled exotic species are on the long term not different from native species, which eventually leads to balanced co-existence of native and exotic species.

Related publications:

- Hummel, H., Wijnhoven, S. (2012). Long-term patterns in the establishment, expansion and decline of invading macrozoobenthic species in the brackish and marine waters of Southwest Netherlands. Marine Ecology: an Evolutionary Perspective (acc.).

- Wijnhoven, S., Escaravage, V., Daemen, E., Hummel, H. (2010). The decline and restoration of a coastal lagoon (LakeVeere) in the Dutch Delta. Estuaries and Coasts 33, 1261-1278.

- Wijnhoven, S., Hummel, H. (2009). Historische analyse exoten in de Zeeuwse delta. De opkomst, verspreiding, ontwikkeling en impact van exoten onder de macrofauna van het zachte substraat in de Zeeuwse brakke en zoute wateren. Report, Monitor Taskforce Publication Series 2009-11.