Acid mine drainage (AMD) is a major environmental issue worldwide. On the West Coast of the South Island, New Zealand, numerous catchments receive AMD, with significant negative impacts on in-stream flora and fauna. Diatoms are commonly regarded as powerful biological indicators and may be found in high abundance in AMD-contaminated streams; however, relatively little work has been done on diatoms in mining environments in New Zealand. Initially, I conducted a survey of epiphytic diatom communities in 39 streams ranging from non-impacted reference streams to those severely impacted by AMD. Streams were assigned to one of four classes along an AMD gradient: circum-neutral reference, naturally acidic reference, moderately impacted, and severely impacted. There was a wide range in diatom taxonomic richness in reference and moderately impacted streams (8 – 33 taxa). Taxonomic richness was greatly reduced in severely impacted streams (1 – 5 taxa) at a threshold of pH 3.4 and was dominated by Pinnularia cf. acidophila (69 – 100% relative abundance). Community composition differed between circum-neutral reference, moderately, and severely impacted streams; however, naturally acidic and moderately impacted streams had similar diatom communities primarily composed of acid-tolerant Eunotia and Frustulia species. This indicated that diatoms are strongly structured by pH and able to tolerate moderate conductivity and metal concentrations without a corresponding shift in community composition. Survey data were then used to develop two diatom-based indices for streams impacted by AMD: a single Biotic Index and a Multimetric Index. While neither index was able to distinguish naturally acidic from moderately impacted streams, both indices successfully categorised streams as circum-neutral reference, moderately or severely impacted by AMD. These indices may be useful in assessing AMD impact on circum-neutral streams or in identifying when a stream has crossed a threshold from moderately to severely impacted by AMD. Diatoms would be especially useful as bioindicators of AMD if they respond rapidly to a change in mine discharge. To test this, mature algal biofilms were reciprocally transferred between circum-neutral reference streams and streams of varying degrees of AMD over a period of 13 days. Diatom mortality increased rapidly in the reciprocal transfer between reference and severely impacted streams. Reference communities resembled the ambient diatom community of severely impacted streams 13 days post-transfer. However, in the reverse transfer, a change in community composition was slow to occur. Diatoms respond faster to an increase in pollution than to pollution amelioration. Overall, results indicated that diatom communities may be a useful tool for monitoring the presence and magnitude of AMD in New Zealand streams.

Abstract As human activities are increasingly affecting natural communities, many communities are impacted by multiple stressors and their interactions. Understanding how natural and anthropogenic stressors act individually and in concert is essential for managing and conserving natural ecosystems efficiently. In this thesis I studied how geology-related natural acidity, land drainage and their interaction affect biological communities and leaf decomposition in boreal headwater streams. I further assessed the concordance of communities along natural and anthropogenic stressor gradients. As model organisms, I used benthic diatoms, bryophytes, invertebrates and leaf-decomposing fungi. I showed that geology-related natural acidity constitutes a strong environmental filter for stream communities, reducing species richness and changing community composition. Community concordance was also generally strongest along the natural acidity gradient. However, natural acidity mostly did not homogenize communities nor did it affect leaf decomposition by fungi. Effects of peatland drainage differed between the two stream types, being mainly sedimentation in the circumneutral streams and increasing metal concentrations in the acid streams. Overall, changes in community composition were better able than pure species richness to track single stressor impacts. Furthermore, fungal assemblages were more homogeneous and decomposition rates slower in drained acidic sites than in any other stream type. Thus the drainage-induced shift in water chemistry in the acidic streams seems to constitute an even stronger environmental filter than sedimentation. Conservation planning needs to give special attention to these vulnerable, naturally stressful ecosystems. / Tiivistelmä Ihmistoiminnan vaikuttaessa yhä enemmän luonnon elinympäristöihin eliöyhteisöihin kohdistuu usein samanaikaisesti monenlaisia paineita. Ekosysteemien tehokas hoitaminen ja suojelu edellyttävät tarkkaa tietoa siitä, miten luonnollinen stressi ja ihmistoiminta yhdessä ja erikseen vaikuttavat ekosysteemeihin. Väitöskirjassani tutkin, kuinka geologiasta johtuva luonnollinen happamuus ja metsäojitus vaikuttavat boreaalisten purojen eliöyhteisöihin ja lehtikarikkeen hajotukseen. Tarkastelin myös eliöryhmien yhdenmukaisuutta ihmistoimintagradientilla ja luonnollisella stressigradientilla. Malliorganismeinani olivat piilevät, vesisammalet, pohjaeläimet ja hajottajasienet. Väitöskirjassani osoitan, että geologiasta johtuva puroveden happamuus toimii merkittävänä ympäristösuodattimena purojen eliöyhteisöille vähentäen lajirunsautta ja muokaten lajistoa. Myös eliöryhmien lajistovaihtelu oli yhdenmukaisinta luonnollisella happamuusgradientilla. Toisaalta luonnollinen happamuus ei vaikuttanut merkittävästi lehtikarikkeen hajotukseen tai purojen väliseen sieniyhteisöjen monimuotoisuuteen. Metsäojituksen fysikaalis-kemiallinen vaikutus erosi purotyypeittäin: pH-neutraaleissa puroissa ojitus pääosin lisäsi pohjan hiekoittumista, kun taas happamissa puroissa veden metallipitoisuudet kasvoivat entisestään. Yleisesti ottaen sekä luonnollisen happamuuden että metsäojituksen vaikutukset näkyivät parhaiten muutoksina eliöyhteisöjen lajikoostumuksessa. Lisäksi ojitetuissa happamissa puroissa hajottajasieniyhteisöjen lajistot olivat keskenään samankaltaisempia ja lehtikarikkeen hajotus hitaampaa kuin muissa purotyypeissä. Metsäojituksen aikaansaama muutos luonnollisesti happamien purojen vesikemiassa näyttää siis olevan jopa merkittävämpi ympäristösuodatin kuin pohjan hiekoittuminen. Luonnollisesti stressattujen elinympäristöjen herkkyys maankäytön muutoksille tulisikin huomioida ympäristön hoidon suunnittelussa nykyistä paremmin.