Tracking environmental change in seagrass meadows: understanding indicator behaviour across space and time

Abstract

Nearshore marine ecosystems like seagrass meadows face a wide range of anthropogenic influences, impacting the system at different spatial and temporal scales. Managing these systems in the face of these pressures requires detailed knowledge of how seagrass habitats respond to these various threats. A plethora of useful indicators have been developed to help managers and policy makers track seagrass meadow health and status, detect environmental impacts or measure the effectiveness of management interventions. However, choosing between these indicators can often be a daunting task since they vary considerably in their overall behaviour in relation to ecosystem and environmental changes. This thesis assesses the most commonly employed seagrass indicators to determine if they are adequate and appropriate to the specific needs of coastal ecosystem management. This assessment is based on evaluating three fundamental characteristics of each indicator – the robustness of its response, the specificity or generality of its response, and the time of response. We use a variety of complementary approaches to explore indicator behaviour. In Chapter 3, we use field-based studies to assess how seagrass indicators respond to the construction of a breakwater in the vicinity of a Posidonia oceanica seagrass meadow. Chapters 4 and 5 examine long-term trends in seagrass indicators to improving water quality conditions after significant regional management interventions. In addition, in Chapter 6, we comprehensively review seagrass indicator responses to multiple stressors. Chapters 3, 4 and 5 focus largely on the Catalan Coast in the Mediterranean with Posidonia oceanica as a target species. Chapter 6 in contrast is a broad review of a wide range of indicators used across several seagrass species worldwide. A central learning across these studies was that the level of biological organisation of the indicator (i.e. Physiological, biochemical, growth, morphological, structural or demographic) is critical in determining the specificity or generality of response: the lower the level (e.g. biochemical), the most specific the response, while the higher the level (e.g. population, community), the wider the response. Thus, biochemical indicators are ideal to determine the identity or even the origin of a pressure while structural indicators, in contrast, are useful as generic indicators of declining conditions. Response times are also heavily determined by the level of organisation, particularly in the detection of improving environmental quality along the Catalan coast. Biochemical indicators responded unequivocally to water quality improvements observed both in the experimental field study (Chapter 3) as well as in the analysis of the long-term data series (Chapters 4 and 5). The meta-analysis confirmed that these trends in specificity and response time were not unique to Posidonia oceanica or the Catalan coast and highlighted the critical role of plant size in determining indicator time responses. Large species take considerably longer to register a response to environmental degradation as well as improvement – a critical factor that needs to be accounted for in designing monitoring programmes and interpreting ecosystem trends. Taken together, these results suggest that differences in the behaviour of seagrass indicators require that they should be carefully selected to match the objectives of management. Based on the results reported in this thesis, where different sets of indicators have been proven successful for given management objectives, we finally develop a simple decision tree to help managers chose the most reliable sets of indicators matching their objectives. Understanding the diversity of responses that seagrass indicators display can make them a powerful set of tools in the ecosystem manager’s toolkit. Carefully employed, they can serve as bespoke solutions to a wide range of management objectives as we seek to monitor and protect these vital ecosystems and coastal water quality in the face of increasing coastal pressures.

Entendre com responen les fanerògames marines a les pressions, és clau per a poder gestionar tant els herbeis com les aigües costaneres. Actualment, disposem d’un gran nombre d’indicadors basats en fanerògames marines. Però, la manca d’informació sobre com responen als canvis ambientals, fa que no sigui fàcil escollir quins indicadors són els més adients per a cada tipus i objectiu de gestió. Aquesta tesi avalua les tres característiques bàsiques de la resposta dels indicadors més utilitzats als canvis ambientals: la robustesa de la resposta, la especificitat dels indicadors a diferents pressions i el temps de resposta. Per analitzar aquestes tres característiques, fem servir diferents aproximacions complementàries. Al capítol 3, analitzem la resposta de diferents indicadors a les obres d’ampliació del port de Blanes, situat just al costat d’un herbei de Posidonia oceanica. Als capítols 4 i 5, estudiem com responen els indicadors a la millora de la qualitat de l’aigua a la costa catalana. Finalment, al capítol 6, presentem una metaanàlisi que estudia com responen els indicadors a diferents factors d’estrès. De totes tres aproximacions, hem pogut comprovar que el nivell d’organització dels indicadors (i.e. bioquímic, estructural) és clau a l’hora de determinar el grau d’especificitat de la resposta dels indicadors a les pressions: generalment, a més baix nivell d’organització (e.g. bioquímic), més específica és la resposta i com més alt (e.g. demogràfic), més ampli és el rang de pressions que un indicador pot detectar. El temps de resposta dels indicadors varia també en funció del nivell d’organització dels indicadors, especialment, quan es tracta de la detecció de millores ambientals. A més a més, la metaanàlisi destaca la importància de la mida de les plantes per determinar el temps de resposta. Les espècies grans triguen més a detectar la degradació de les condicions ambientals i, molt més, a detectar la millora, especialment, si s’utilitzen indicadors estructurals o demogràfics. Basant-nos en els resultats d’aquesta tesi, hem elaborat un esquema per ajudar els gestors a escollir el conjunt d’indicadors que més s’ajusti a cada objectiu de gestió. Utilitzats correctament, aquests indicadors són molt útils per fer el seguiment, tant de l’estat de salut dels herbeis, com de la qualitat del medi.