Effects of nutrient enrichment on seagrass population dynamics: evidence and synthesis from the biomass–density relationships

Abstract

Summary 1.The available data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment were analysed to assess the intraspecific mechanisms operating within seagrass populations and whether biomass-density relationships can provide relevant metrics for monitoring seagrasses.2. The response of shoot biomass and density to nutrient enrichment was dependent on the type of study; the short-term positive response of biomass and density in experimental studies reveals context-specific nutrient limitation of seagrasses. The long-term negative response of descriptive studies probably results from ecosystem-scale events related to nutrient enrichment such as increased turbidity, algal blooms, epiphyte loads and anoxia.3. Most seagrass species analysed lie in the nonthinning part of the theoretical biomass-density curves. A simultaneous increase in biomass and decrease in density, evidence of self-thinning, were only observed in 4 of 28 studies. The analysis of both the static and the dynamic biomass-density relationships revealed that the slopes increase under nutrient enrichment. Surprisingly, the species-specific slopes (log B-log D) were higher than one, revealing that the B/D ratio, that is, the average shoot biomass, increases with density in all seagrass species analysed. Nutrient enrichment further enhanced this effect as biomass-density slopes increased to even higher values. The main drivers behind the increasing biomass-density slopes under nutrient enrichment were the increase in shoot biomass at densities above a species-specific threshold and/or its decrease below that threshold. Synthesis.Contrasting short-and long-term responses of both biomass and density of seagrasses to nutrient enrichment suggest that the former, positive ones result from nutrient limitation, whereas the later, negative ones are mediated by whole ecosystem responses. In general, shoot biomass of seagrasses increases with density, and nutrient enrichment enhances this effect. Experimental testing of facilitation processes related to clonal integration in seagrasses needs to be done to reveal whether they determine the low incidence of self-thinning and the intriguing biomass-density relationships of seagrass species. The increasing slopes and decreasing Journal of Ecology 2013Ecology , 101, 1552Ecology -1562Ecology doi: 10.1111Ecology /1365Ecology -2745 intercepts of the species-specific dynamic biomass-density relationships of seagrasses and the decreasing coefficients of variation of both biomass and density constitute relevant, easy-to-collect metrics that may be used in environmental monitoring.

Atmospheric Nitrogen Deposition Increases the Possibility of Macroalgal Dominance on Remote Coral Reefs

“…Additionally, little promoted effect on macroalgae was observed (Figure ), despite elevated nutrients (3.15 μmol/L) throughout the whole water column. Comparable results have been reported in other studies showing little promotion or even inhibition on seagrasses because coastal nutrient enrichment led to increased turbidity and enhanced light limitation (Cabaço et al, ). The contrasting growth response to coastal and remote environments suggests that atmospheric N has a greater impact on remote reefs.…”

Biomass–density data analysis: a comment on Cabaço et al. (2013)

“…1 in Cabaco et al . ()). We performed an optional first step in DFA measuring the Mahalanobis distance from each of the observations to the groups’ centres, having found 8 of the 28 observations (28.57%) were actually closer to the centre of the opposite group (Tt d , Tt e and Zc b in experimental studies and Tt b , Tt c , Cs, Pa and Si in descriptive studies).…”

“…The data used for re‐analysis in this article was retrieved by directly measuring the data from screen‐magnified Figs 1 and 2 in Cabaco et al . ().…”

“…In a recent paper, Cabaço et al . () analyse data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment, to (i) evaluate the intraspecific mechanisms operating within populations and (ii) determine whether biomass–density relationships can provide relevant metrics for monitoring. We contend that there are serious flaws in the data analyses and interpretation leading to incorrect or unsustained conclusions as they stand in the paper.…”

Seagrass meadows are threatened by expected loss of peatlands in Indonesia

“…Some reef seagrass meadows in Indonesia far from land will be nutrient poor, and therefore, increased DIN from peatland degradation will be of benefit resulting in increased growth, but most seagrass meadows (especially coastal and estuarine meadows) in Indonesia are probably already in states of elevated DIN and are at risk. We also know that many short‐term experimental enrichment studies on nutrient‐limited seagrass result in short‐term increases in biomass, but long‐term studies find elevated nutrients to result in declining seagrass biomass and density (Cabaço et al ., ). The model created by Abrams et al .…”