Long-term Patterns of Eelgrass (Zostera marina) Occurrence and Associated Herbivorous Waterbirds in a Danish Coastal Inlet

Abstract

Seagrasses in coastal areas have substantial importance for the marine environment and also serve as food for herbivorous waterbirds. We investigated potential relationships between the autumn population of herbivorous waterbirds and eelgrass (Zostera marina) abundance in the EU protected area, Nibe-Gjøl Bredning, a broad of the Limfjorden estuarine complex in Denmark.This is an important site for migratory herbivorous waterbirds such as mute swan (Cygnus olor), coot (Fulica atra), brent goose (Branta bernicla), and wigeon (Anas penelope). We explored long-term (27 years) changes in eelgrass and bird-populations and relationships between eelgrass-and bird abundance. We applied trend-and correlative analyses of yearly monitoring data on eelgrass and waterbirds between 1989 and 2015 coupled with estimates of the potential grazing pressure exerted by the birds. Around 1990 eelgrass was abundant in this area covering more than 40 km 2 , but eelgrass coverage and biomass declined drastically around 1995 and remained low until 2011 when natural recolonization accelerated and by 2015 had restored the lost meadows. The number of herbivorous waterbirds also fluctuated substantially during the monitoring period with large abundance until the mid-end 1990s followed by reduced abundance in the 2000s and recovery after 2010. The number of bird-days showed a positive relationship with the same year's eelgrass abundance in the 1-2 m depth stratum. For the 0-1 m depth stratum, where the eelgrass meadows are most exposed to bird grazing but also to physical control from e.g., wind and ice, only a particularly detailed eelgrass data set available for a subset of the study period, showed a significant relationship with bird grazing. The potential waterbird consumption of eelgrass, estimated by multiplying average intake rate and number of bird-days for each species, ranged from less than 16% of the eelgrass biomass in most years to more than 40% of the eelgrass biomass in years with extremely sparse eelgrass populations. Hence, the study suggests that dense eelgrass populations stimulate herbivorous waterbirds whereas top-down control is only likely when abundant bird populations graze on sparse eelgrass populations.

Seagrass meadow cover and species composition drive the abundance of Eurasian wigeon (Mareca penelope L.) in a lagoon ecosystem of the northern Adriatic Sea

“…As expected the overall number of individuals of wigeon increased where the seagrass meadows were larger. This confirms the finding of other studies (Balsby et al, ; Hamza, Hammouda, & Selmi, ), where it is reported that dense seagrass populations favor waterbirds, especially in large intertidal areas, characterized by high numbers of tidal channels, elevated amounts of mud and organic materials in the sediment. In such situations, coverage of seagrass hosted a great diversity of species, sustaining also wigeon flocks.…”

Meta-Analysis of Reciprocal Linkages between Temperate Seagrasses and Waterfowl with Implications for Conservation

“…Small scale studies (within bay or within year): , Dixon, 2009 ; , Percival and Evans, 1997 ; , Tinkler et al, 2009 . Large scale studies (across bays or years): ×, Wilson and Atkinson, 1995 ; ∗ , Percival et al, 1998 ; , Balsby et al, 2017 ; , Petersen et al, 2008 ; , Dos Santos et al, 2012 ; , Moore et al, 2004 . Raw correlations between bird and Zostera abundances for each study are provided in Supplementary Figure S2 .…”

“…With respect to annual production, estimates of percent consumed by birds are generally low: e.g., 4% ( Nienhuis and Groenendijk, 1986 ). However, estimates relative to standing biomass, winter productivity, or carrying capacity are frequently larger: 40% ( Percival et al, 1998 ), 45% ( Nacken and Reise, 2000 ), 50% ( Jacobs et al, 1981 ; Baldwin and Lovvorn, 1994 ), 16–73% ( Balsby et al, 2017 ), 80% ( Hori and Hasegawa, 2005 ; Inger et al, 2006a ), and total removal of 100% ( Vermaat and Verhagen, 1996 ). Therefore, it appears possible for birds to have strong top-down effects seasonally even if this removal is a small fraction of total annual production, and reinforces that herbivory on Zostera by birds tends to be concentrated in time and space.…”

“…We calculated effect size using Pearson’s correlation coefficient ( r ) between waterfowl and Zostera , followed by transformation with Fisher’s Z ( ) and a calculated standard deviation derived from sample size ( ). We calculated an effect size for each waterfowl species separately if the study reported multiple species ( Percival et al, 1998 ; Petersen et al, 2008 ; Balsby et al, 2017 ).…”

“…FIGURE S2 | Relationships between bird and Zostera abundance. (A) Balsby et al (2017) , (B) Petersen et al (2008) , (C) Moore et al (2004) , (D) Wilson and Atkinson (1995) , (E) Percival et al (1998) , (F) Tinkler et al (2009) , (G) Clausen and Percival (1998) , (H) Dos Santos et al (2012) , (I) Dixon (2009) . Data from Percival and Evans (1997) are presented in their Figure 4.…”