Millennial-scale trends and controls in Posidonia oceanica (L. Delile) ecosystem productivity

Abstract

Posidonia oceanica is a marine phanerogam that buries a significant part of its belowground production forming an organic bioconstruction known as mat. Despite Posidonia seagrass mats have proven to be reliable archives of long-term environmental change, palaeoecological studies using seagrass archives are still scarce. Here we reconstruct four millennia of environmental dynamics in the NE coast of Spain by analysing the carbon and nitrogen stable isotopic composition of P. oceanica sheaths, the proportion of different seagrass organs throughout the seagrass mat and other sedimentological proxies. The palaeoenvironmental reconstruction informs on long-term ecosystem productivity and nutrient loading, which have been linked to global (e.g., solar radiation) and local (e.g., land-use changes) factors. The long-term environmental records obtained are compared with previous palaeoecological records obtained for the area, showing a common environmental history. First, a relative seagrass ecosystem stability at ~4000 and 2000 cal. yr BP. Then, after a productivity peak at ~1400-800 cal. yr BP, productivity shows an abrupt decline to unprecedented low values. The fluctuations in ecosystem productivity are likely explained by increases in nutrient inputs related to human activitiesmostly in the bay watershedconcomitantly with changes in total solar radiation. Cumulative anthropogenic stressors after Roman times may have started to affect ecosystem resilience, dynamics and productivity, with more abrupt regime shifts during the last millennium. These results add into recent research showing the potential of seagrass archives in reconstructing environmental change and seagrass post-disturbance dynamics, hence providing unvaluable information for improving the efficiency in managing these key coastal ecosystems.

Pedogenic Processes in a Posidonia oceanica Mat

“…These small changes likely reflect past changes in the proportion of mineral and organic inputs to the soil of the meadow. The main change observed is a 15-fold excursion to positive values at 300 cm depth (Figure 3), concurring with a high erosion phase in the bay catchment led by an increase in flooding events [62] and human activity, which is in agreement with environmental paleoreconstructions of the area [34,43].…”

“…The third principal component scores show fairly constant values below 35 cm, with the exception of relatively lower values between 200 cm and 300 cm (CP3.2), suggesting a coarser grain size composition (Figure 3). It is interesting to note that this decrease in fine mineral particles is coeval with the peak in inorganic mineral content (PC1 scores, CP1.2, Figure 3), reinforcing the occurrence of an intense 'terrestrial event' already discussed elsewhere [34,43]. Again, a high accumulation of plant remains in the upper part of the soil and could account for the large decrease in Fe and Ti concentrations (CP3.1).…”

“…Although information about the biogeochemistry of the substrate of the Portlligat meadow is scarce, it is by far the best-studied seagrass deposit below 50 cm from the sediment top, as it has been used as a source of paleo-environmental information [31,33,34,43,50]. The average thickness of the deposit has been estimated to be 4.5 m, accumulated in the last 6000 years [41].…”

“…(a) Coring site of the P. oceanica soil core in Portlligat Bay (Girona, Spain); (b) detail of the upper and lower sections of the core, where plant fibers can be found. Modified from Leiva-Dueñas (2018)[43].…”

Factors regulating primary producers' assemblages in Posidonia oceanica (L.) Delile ecosystems over the past 1800 years

“…Therefore, elucidation of the long-term dynamics and environmental drivers of ecosystem change is required to evaluate the magnitude of current declines in an historical context, as well as to predict, prevent or mitigate the effects of present and future environmental changes on seagrass meadows structure and function ( López-Merino et al, 2017;Leiva-Dueñas et al, 2018).…”