Substrate Heterogeneity as a Trigger for Species Diversity in Marine Benthic Assemblages

Abstract

Many studies show that habitat complexity or habitat diversity plays a major role in biodiversity throughout different spatial scales: as structural heterogeneity increases, so does the number of available (micro-) habitats for the potential species inventory. The capability of housing species (even rare species) increases rapidly with increasing habitat heterogeneity. However, habitat complexity is not easy to measure in marine sublittoral sediments. In our study, we came up with a proposal to estimate sublittoral benthic habitat complexity using standard underwater video techniques. This tool was subsequently used to investigate the effect of habitat complexity on species richness in comparison to other environmental parameters in a marine protected area situated in the Fehmarn Belt, a narrow strait in the southwestern Baltic Sea. Our results show that species richness is significantly higher in heterogeneous substrates throughout all considered sediment types. Congruently, the presence of rare species increases with structural complexity. Our findings highlight the importance of the availability of microhabitats for benthic biodiversity as well as of the study area for regional ecosystem functioning.

Keywords: Baltic Sea; habitat complexity; macrozoobenthos; rare species; species richness.

Figure 1

Map including the expected distribution of sediment types (after Tauber, 2012 [24]) and the location of sampling stations. Circles indicate 162 sampling locations (further referred to as stations) used for statistical analyses and the construction of general linear models, with circle colors referring to the results of the sediment type classification (crosses indicate samples excluded from the initial dataset (n = 355) due to the criteria listed in Section 2.2). The blue line indicates the border of the Natura 2000-site and the white area indicates unmapped seabed in Danish waters.

Figure 2

Composition of collected organisms. Polychaetes, molluscs, and crustaceans contributed most to species richness.

Figure 3

Boxplot showing species richness per grab and sediment type (mud, fine sand, medium sand, and coarse substrate). Boxes indicate the 25–75% interval, whiskers the 5–95% interval. Note that the black lines represent the median values that differ from the mean. Corresponding number of samples per category are given above each bar. Outliers are marked with circles.

Figure 4

Species area curves for the four sediment types: mud (n = 53), fine sand (n = 27), medium sand (n = 53), and coarse substrate (n = 29), with vertical bars indicating the confidence interval at each step. A sample covers 0.1 m².

Figure 5

Boxplot showing species richness per grab and substrate heterogeneity class, as defined in Table 1. Boxes indicate the 25–75% interval, whiskers the 5–95% interval. Note that the black lines represent the median values that differ from the mean.

Figure 6

Boxplot showing species richness per grab separately for sediment types mud, fine sand (top line), medium sand, and coarse substrate (bottom) per substrate heterogeneity class. Boxes indicate the 25–75% interval, whiskers the 5–95% interval. Note that the black lines represent the median values that differ from the mean.